The image over illustrates the ten crucial functions of windy health. Public health plays a an especially important role for 3 of the functions: monitoring, investigating, and evaluating. The 10 vital Public wellness Services explain the publicly health activities that all areas should undertake. Public wellness systems should

Monitor wellness status to identify and solve ar health problems.Diagnose and investigate health and wellness problems and also health hazards in the community.Inform, educate, and empower people about health issues.Mobilize ar partnerships and activity to identify and also solve wellness problems.Develop policies and plans that assistance individual and community health efforts.Enforce laws and also regulations that safeguard health and also ensure safety.Link world to needed personal health services and assure the supplication of health treatment when otherwise unavailable.Assure competent public and an individual health care workforce.Evaluate effectiveness, accessibility, and also quality of personal and population-based health services.Research for brand-new insights and also innovative remedies to health and wellness problems.

You are watching: Epidemiology searches for associations between exposures and:

Disease surveillance systems and health data sources administer the raw information important to monitor fads in health and disease. Descriptive epidemiology provides a means of organizing and analyzing these data in stimulate to know variations in an illness frequency geographically and over time, and also how an illness (or health) varies among people based on a host of an individual characteristics (person, place, and also time). This makes it feasible to identify trends in health and an illness and also provides a way of planning resources for populations. In addition, descriptive public health is necessary for generating hypotheses (possible explanations) about the components of health and disease. Through generating hypotheses, descriptive epidemiology also provides the starting point because that analytic epidemiology, i m sorry formally test associations between potential determinants and also health or condition outcomes. Details tasks the descriptive epidemiology are the following:

Monitoring and reporting ~ above the health status and health related habits in populationsIdentifying arising health problemsAlerting united state to potential risks from bioterrorismEstablishing public health priorities for a populationEvaluating the effectiveness of treatment programs andExploring potential associations between "risk factors" and health outcomes in bespeak to generate hypotheses about the determinants of disease.Learning Objectives

After effectively completing this unit, the student will be maybe to:

Explain the duty of descriptive researches for identifying problems and also establishing hypotheses.Explain how the features of person, place, & time are supplied to formulate hypotheses in acute disease outbreaks and in researches of chronic diseases.Identify situation reports and also case collection and describe their uses and also their limitations.Describe the style features of one ecologic study and discuss your strengths and also weaknesses.Explain the ide of ecologic fallacy both in general and in the context of a study. Identify the strengths and also limitations of one ecologic study.Describe the architecture features the a cross-sectional study and describe their uses, strengths, and limitations.


Hypothesis Formulation – characteristics of Person, Place, and also Time

Descriptive epidemiology searches for fads by analyzing characteristics the person, place, & time. These characteristics are closely considered when a an illness outbreak occurs, since they provide important clues concerning the resource of the outbreak.

Hypotheses about the factors of disease arise from considering the qualities of person, place, and also time and also looking because that differences, similarities, and correlations. Take into consideration the adhering to examples:

Differences: if the frequency of an illness differs in 2 circumstances, it might be caused by a factor that differs between the two circumstances. For example, there to be a comprehensive difference in the incidence that stomach cancer in Japan & the US. Over there are likewise substantial distinctions in genetics and diet. Probably these determinants are related to stomach cancer.Similarities: if a high frequency of condition is discovered in several different circumstances & one can identify a typical factor, then the common factor may be responsible. Example: AIDS in IV drug users, recipients the transfusions, & hemophiliacs suggests the possibility that HIV can be transmitted via blood or blood products.Correlations: If the frequency of an illness varies in relation to part factor, climate that element may it is in a reason of the disease. Example: distinctions in coronary heart condition vary through cigarettes consumption.

Descriptive public health provides a method of organizing and analyzing data on health and wellness and disease in bespeak to understand variations in disease frequency geographically and over time and how an illness varies amongst people based upon a organize of an individual characteristics (person, place, and also time). Epidemiology had its beginnings in the desire to understand the determinants of acute transmittable diseases, yet its methods and also applicability have broadened to encompass chronic diseases as well.

Descriptive public health for Infectious disease Outbreaks

Outbreaks generally come to the fist of state or local wellness departments in one of two ways:

Astute individuals (citizens, physicians, nurses, activities workers) will sometimes notice cases of condition occurring close in addition to respect to time and/or location or castle will notice several people with unusual functions of condition and report them to wellness authorities.Public wellness surveillance systems collect data ~ above "reportable diseases". Requirements for reporting transmittable diseases in Massachusetts are described in 105 CMR 300.000 (Link to Reportable Diseases, Surveillance, and also Isolation and also Quarantine Requirements).

Clues around the resource of an episode of transmittable Disease

When an outbreak occurs, one of the very first things that must be thought about is what is known about that certain disease. How deserve to the an illness be transmitted? In what settings is it frequently found? What is the incubation period? There space many great summaries easily accessible online. For example, Massachusetts DPH offers this attach to a PDF fact sheet for Hepatitis A, which carry out a an extremely succinct summary. V this background details in mind, the initial task is to start to characterize the cases in terms of an individual characteristics, location, and time (when did they become ill and also where could they have actually been exposed given the incubation duration for the disease. In sense, we are looking for the common facet that defines why every one of these people became ill. What do they have in common?


Information around the cases is typically recorded in a "line listing," a grid on which details for each case is summarized through a separate pillar for each variable. Demographic information is constantly relevant, e.g., age, sex, and address, because they are often the characteristics most strongly related to exposure and to the hazard of disease. In the beginning of an examination a small variety of cases will be interviewed to look because that some common link. This are described as "hypothesis-generating interviews." depending upon the means by i beg your pardon the condition is generally transmitted, the investigator might additionally want come know about other an individual characteristics, such together travel, occupation, recreation activities, use of medications, tobacco, drugs. What did these victims have in common? wherein did they execute their grocery store shopping? What restaurants had they unable to do to in the past month or so? had actually they traveled? had actually they to be exposed come other people who had actually been ill? Other characteristics will be an ext specific to the condition under investigation and also the setting of the outbreak. Because that example, if you were investigating an episode of hepatitis B, you should consider the usual high-risk exposures for that infection, such together intravenous drug use, sexual contacts, and health care employment. The course, with break out of foodborne disease (such as hepatitis A), it would be essential to ask numerous questions about possible food exposures. Where execute you normally eat her meals? carry out you ever eat in ~ restaurants or achieve foods indigenous sources outside the home? theory generating interviews may conveniently reveal part commonalities that administer clues about the feasible sources.


Assessment of an episode by place provides information on the geographic extent of a problem and also may also show swarm or trends that provide clues to the identity and origins the the problem. A basic and useful an approach for feather at geographical patterns is to plot, top top a "spot map" of the area, wherein the influenced people live, work, or may have been exposed. A spot map of instances may show clusters or patterns that reflect water supplies, wind currents, or proximity to a restaurant or grocery store store.

In 1854 there to be an epidemic of cholera in the broad Street area of London. John Snow established the residence or ar of business of the victims and also plotted castle on a street map (the stacked black color disks ~ above the map below). He detailed that the cases were clustered around the large Street community pump. That was additionally noteworthy that there were large numbers of workers in a regional workhouse and also a brewery, but none the these workers were influenced - the workhouse and also brewery each had their very own well.


On a point out map in ~ a hospital, education home, or other such facility, clustering usually suggests either a focal resource or person-to-person spread, when the scattering of instances throughout a basic is much more consistent with a common source such as a dining hall. In studying an outbreak of operation wound epidemic in a hospital, we can plot instances by operation room, restore room, and ward room to look because that clustering.

Link to more on the outbreak that cholera in the vast Street area that LondonLink come an enlarged variation of Snow"s spot map


When investigating the source of break out of transmittable disease, Investigators document the date of beginning of condition for every of the victims and also then plot the start of brand-new cases in time to create what is referred to as one epidemic curve. The epidemic curve for break out of hepatitis A is presented in the illustration below. Begriming in late April, the number of brand-new cases rises come a height of twelve brand-new cases report on may 12, and also then the number of brand-new cases progressively drops ago to zero by might 21. Knowing that the incubation duration for hepatitis A averages about 28-30 days, the investigators concluded the this to be a point resource epidemic due to the fact that the swarm of brand-new cases all arisen within the expectations of a single incubation duration (see explanation ~ above the following page). This, in connect with various other information, listed important clues that assisted shape their hypotheses around the source of the outbreak.

Video Summary: Person, Place, and also Time (10:42)

Epidemic Curves

An "epidemic curve" shows the frequency of brand-new cases end time based upon the date of onset of disease. The shape of the curve in relation to the incubation duration for a particular condition can provide clues around the source. There are three straightforward types of epidemic curve.

Point source outbreaks (epidemics) show off a typical source, such together contaminated food or an infected food handler, and also all the exposures often tend to take place in a reasonably brief period. Consequently, point source outbreaks have tendency to have epidemic curves with a rapid increase in cases followed by a somewhat slower decline, and every one of the instances tend to loss within one incubation period. The graph over from a hepatitis outbreak is an instance of a point source epidemic. The incubation period for hepatitis varieties from 15-50 days, through an typical of around 28-30 days. In a point resource epidemic of hepatitis A girlfriend would mean the rise and also fall of new cases to take place within about a 30 day expectancy of time, i beg your pardon is what is seen in the graph below.

Continuous common resource epidemics may also rise to a peak and then fall, however the cases do not all happen within the expectancy of a single incubation period. This indicates that over there is an ongoing source of contamination. The under slope of the curve may be really sharp if the common resource is removed or gradual if the outbreak is enabled to exhaust itself. The epidemic curve listed below is native the cholera outbreak in the vast Street area the London in 1854 the was investigated by Dr. John Snow. Cholera has an incubation period of 1-3 days, and also even though residents started to flee as soon as the outbreak erupted, you have the right to see that this outbreak it is long for an ext than a solitary incubation period. This argues an ongoing resource of infection, in this situation the vast Street pump.


Propagated (or progressive source) epidemic. The epidemic curve shown below is from break out of measles that started with a single index case who infected a number of other individuals. (The incubation period for measles averages 10 days v a range of 7-18 days.) One or much more of the civilization infected in the initial wave infected a team of human being who come to be the second wave the infection. So here transmission is person-to-person, fairly than indigenous a common source. Propagated epidemic curve usually have actually a collection of successively bigger peaks, which are one incubation duration apart. The succeeding waves tend to involve an ext and an ext people, till the pool of susceptible world is worn down or manage measures room implemented. This is perfect example, however; in reality, many of this epidemics carry out not create the classic pattern.


For part outbreaks the descriptive information is all that is needed to number out the source, and control measures have the right to be undertaken rapidly. In other cases, this descriptive information (person, place, and time) helps create hypotheses around the source, yet it isn"t obvious what the source is. When this occurs, the is important to test the hypotheses through conducting an analytical study, i.e. Either a case-control research or a cohort study. This means collecting data and assessing it in bespeak to determine the source. After the hepatitis outbreak in Marshfield, DPH performed a case-control study. After break out of Giardia in Milton, MA, a retrospective cohort examine was conducted. However, the is essential to identify that you can"t check a hypothesis unless you have one to test. So, the descriptive studies that generate hypotheses space essential.

Use the graph listed below to prize this "Quiz Me."


(Optional) -Two approaches for creating an Epidemic Curve in Excel

Method 1 - video

Method 2 - video

(Optional) - measures in the examination of a disease Outbreak

Most outbreak investigate involve the following steps:

Preparation for the investigationVerifying the diagnosis and also establishing the visibility of an outbreakEstablishing a case meaning and finding casesConducting descriptive epidemiology to determine the an individual characteristics of the cases, alters in an illness frequency end time, and also differences in disease frequency based upon location.Developing hypotheses about the cause or sourceEvaluating the hypotheses & refining the hypotheses and conducting extr studies if necessaryImplementing control and prevention measuresCommunicating the findings

Some of these steps might be carried out simultaneously, and the order might vary relying on the circumstances. For example, if brand-new cases are proceeding to occur and there are steps that deserve to be taken to control the outbreak and also prevent much more cases, then certainly control and prevention actions would take height priority.

Optional added Resources

General info on Outbreak Investigations

For an introduction of outbreak investigations for foodborne disease see the CDC internet page linked here. Other good general resources of details on exactly how to conduct outbreak investigations have the right to be found in the college of phibìc Carolina (UNC) online focus on Field epidemiology series. The complying with links to online posts may it is in of interest:

Volume 1

Issue #1: rundown of Outbreak Investigations

Issue #2: Anatomy and Physiology of an outbreak Team

Issue #3: Embarking on an outbreak Investigation

Issue #4: instance Finding and also Line Listing: A overview for Investigators

Issue #5: Epidemic curves Ahead through a focus Flash on producing an Epidemic Curve in Excel

Issue #6:Hypothesis Generation during Outbreaks

Volume 2:

Issue #1: Hypothesis-Generating Interviews

Issue #2: developing a Questionnaire

Issue #3: Interviewing Techniques

Another great general resource is "Hepatitis in Sparta." This is an online interactive teaching situation that thrusts the student into the role of investigator make the efforts to determine the source for an outbreak of hepatitis instances in the city of Sparta.

Descriptive epidemiology for Chronic Diseases

The exact same questions around person, time, and place deserve to be applied to chronic diseases. Who space the world who have the disease? What space their characteristics? What is your occupation? Where execute they live and also work? just how did condition occurrence vary over time?

Personal Characteristics

Personal attributes also provide clues around the reasons of chronic diseases. Many condition vary in relation to age and also gender, however many other attributes are additionally important, such as occupation, diet, sexual activity, take trip history, and personal behaviors (exercise, smoking, etc.)

Age-specific rates of Disease

Because so numerous diseases vary in relationship to disease, one frequently sees condition rates categorized this way - so-called "age-specific rates of disease." Mortality prices are an extremely low in the youngest age groups & comparable in males and females. In adulthood the mortality rates rise sharply and also become higher in males. Return the mortality rate proceeds to rise into old age, the sex difference starts to narrow. One can describe this as a chronic, progressive condition in i beg your pardon the gender differences raise the concern of even if it is sex hormone play a role, specifically since females begin to capture up after ~ menopause occurs.

Table - death Rates native Coronary Artery an illness (Age-Specific Rates)

Age team (years)






























Differences by Race and Ethnicity

In addition to age and also gender one might want to research how an illness rates differ with respect to various other characteristics, such together race. The table below summarizes. Yearly mortality rates per 100,000 in whites and blacks in the United claims in 1967. Ethnic and racial differences in condition rates sometimes have actually a hereditary basis, e.g., sickle cell anemia in people of african descent or beta thalassemia in people of Mediterranean descent, however in other instances racial distinctions are as result of environmental or socioeconomic factors.

Link to an ext on sickle cabinet anemiaLink to an ext on beta thalassemia

Table - yearly Mortality rates per 100,000 populace in the US, 1967






Hypertensive love disease

Diabetes mellitus


Non-motor vehicle accidents

Motor car accidents

Cirrhosis that liver

Respiratory cancer






















Other personal Characteristics

Besides age, gender and race/ethnicity, other personal characteristics that might be important to take into consideration are:

OccupationDietReligious practices, e.g. Dietary constraints or restrictions on drinking alcohol or tobacco useLeisure activities, e.g., exercise

Place: sport by Location

Differences in disease frequency by location provides vital clues around the components of chronic diseases. Wherein does the condition tend to occur?

Does the frequency of an illness vary from country to country? Or state come state?Does it vary amongst cities or neighborhoods?Does it differ within different parts the a large workplace?

Example 1: Stomach Cancer by ar in the US

These maps show fatality rates from stomach cancer in females (top) and males (below) in different US counties. The darkness the shading of every county indicates exactly how its stomach cancer rate compares with the nationwide average. The darkest shading indicates rates well over average, and white shading indicates rates listed below average; the gray shading indicates intermediate levels. Keep in mind that rates of stomach cancer often tend to be high in counties in the north-central part of the nation in both males and also females. Investigators speculated that these clusters might correlate with populaces of German or Scandinavian descent who have actually a heritage of eating smoked fish. Might the high prices of stomach cancer be the result of their intake of exhilaration fish or other traditional methods of food preservation?


Source: Atlas the Cancer Mortality because that U.S. Counties: 1950-1969, TJ Mason et al, PHS, NIH, 1975

Example 2:Differences in prices of Stomach Cancer in Japan and also US

Rates that stomach cancer likewise vary amongst countries. Japanese have actually a greater rate of stomach cancer 보다 Caucasians in California. Is this as result of a hereditary difference? A dietary difference? The rate amongst Japanese human being diminishes after ~ they move to US, and also diminishes even more in their offspring. One possibility is that once the Japanese relocate here, they start to transition to one American diet, and this trend is also stronger in their children. Room there necessary dietary differences? Could usage of huge amounts of acting fish be a cause of stomach cancer?


Mortality Rate

(per 100,000 population)

Japanese in Japan

Japanese immigrant to California

Sons that Japanese immigrants

Native Californians (Caucasians)





Variation in an illness Over Time

Has the frequency of disease changed over number of decades?Does frequency of an illness vary in a cyclic means that relates to the seasons?Has it changed over the food of days?

Changes in disease rate over time have the right to also provide clues for chronic diseases.

Example 1: yearly Mortality native Pulmonary Tuberculosis in England and also Wales

Tuberculosis (TB) is just one of the great killers of all times. The graph top top the right mirrors the mortality price from TB native 1855-1955 in England and also Wales. The impressive downward trend started well before the advancement of antibiotics. The steady improvement was probably a direct an outcome of "the restroom idea" which brought about concerted initiatives to enhance working and living conditions, nutrition, ventilation, and waste management. Also, note the boosts in TB mortality that arisen during human being War I and World battle II. This suggests that nutritional deficiencies, translocation, crowding, and other adverse circumstances associated with war room contributing components to the causation of TB.


Example 2: toxicity Shock and Rely Tampons

In January 1980 there were numerous reports of toxicity shock syndrome due to infection v Staphylococcus aureus bacteria, and the descriptive epidemiology indicated the the problem was occurring primarily in menstruating women. A CDC task force investigated and also eventually traced the outbreak come the arrival of count tampons, a super absorbent product marketed through Proctor and also Gamble. The monthly situations of toxicity shock syndrome in 1980-1981 are presented in the graph below . The graph mirrors that prior to 1978 there were just occasional cases of toxic shock syndrome in the joined States. After count tampons were presented in 1978, there to be a steady rise in toxicity shock situations which peaked at around 125 per month in 1980. Shortly after that, rely tampons to be taken turn off the market, and also the incidence decreased sharply.


There were actual two pieces of proof related to time variations that supported rely tampons as the cause. First, descriptive epidemiology suggested a connect to menstruation, leading doctors to take bacterial cultures from the vagina. This noted a an essential clue saying a attach to details brands the tampons. In addition, the frequency of toxicity shock syndrome plainly correlated through the introduction and succeeding removal of rely tampons from the market.

Link to more on toxicity shock syndrome

Other determinants That deserve to Produce alters in condition Frequency end Years or Decades

If the frequency that a disease or mortality indigenous a condition changes over time, there are several components which could be responsible:

Changes in incidence due to environmental or life-style changes.Improvements in diagnosis may increase instances reported also though the incidence may not it is in changing.Changes in record keeping (accuracy) can produce what appear to be changes in condition rates.Improved treatment might decrease mortality ratesChanges in the age circulation of a population can produce transforms in the as whole rate the disease, even though age-specific prices are no changing.

Two an essential Types of research Questions

Specifying the research inquiries is crucial to selection of an proper study population, and also infinite concerns exist. Nevertheless, Keyes and Galea stress and anxiety two basic types of research questions which have essential implications selecting an suitable study design.

These are:

1. Inquiries whose goal is precise estimation of population parameters


What ratio of high school students smoke? Or usage drugs?What is the frequency of fatality from coronary artery condition among black and also white males and females, and also how have actually those rate changed over the past 20 years?

Questions favor these need samples that are representative the the population gift studied, that is comparable to the population in their qualities (and they require adequate sample dimension in stimulate to minimize sampling error).

2. Inquiries whose score is to identify and also quantify exposures that have causal effects on health and wellness outcomes.


Does usage of cell phones cause cancer?Do "brain exercises stop cognitive decline with advancing age?Do childhood vaccinations reason autism?

Questions like these also require an adequate sample size to exactly assess the magnitude of an effect, however they different from concerns aimed at parameter estimation in that that they need making comparisons, e.g., comparing risk in between exposed and also non-exposed persons. As soon as trying come answer questions like these regarding etiology, the is not so necessary that the samples it is in representative the the in its entirety population, but for exact assessment the the result the groups being contrasted must be equivalent to each other with respect to other factors that impact the outcome.

Fundamental study Designs because that Both Representative and also Purposive Studies

Keyes and also Galea determine three an essential approaches to study architecture that can be used regardless of whether one"s score is to take it representative samples to estimate population parameters or to take purposive samples in stimulate to recognize whether a offered exposure or factor causes one or much more health outcomes.

One have the right to study the sample at a certain point in time.One can follow the sample forward over time to compare the frequency of wellness indicators among two or more exposure groups.One can examine the retreat exposure background of a sample.

The 2nd option will only be made use of in analysis studies, which will be covered in a separate module, yet the an initial two options will be checked out in the next section explicate several varieties of descritive studies.

Categories of Descriptive Epidemiology

Case Reports

A instance report is a detailed summary of an illness occurrence in a single person. Unusual functions of the case may indicate a new hypothesis around the reasons or instrument of disease.

Example: acquired Immunodeficiency in one Infant; possible Transmission by method of Blood Products

Link to write-up by Ammann AJ et al: obtained immunodeficiency in an infant: possible transmission by means of blood products. The Lancet 1:956-958, 1983.

In April 1983 it had not yet been presented that AIDS could betransfer by blood or blood products. An child born with Rh incompatibility; compelled blood assets from 18 donors end 8 weeks and also subsequently arisen unusual recurrent infections v opportunistic agents such together Candida. The infant"s T cell counting was low, saying AIDS. There was no family background of immunodeficiency, but one that the blood donors was discovered to have died of AIDS. This led the investigators come hypothesize the AIDS might betransfer by blood transfusion.

Example: Survival after therapy of Rabies with Induction that Coma.

Link to write-up by Willoughby R, Jr., et al: N Engl J Med 2005;352:2508-14.

Rabies is practically uniformly fatal as soon as it develops. As of 2005 there had been only 4 survivors, every of whom got rabies prophylaxis ~ the bite, but before symptoms developed. Willoughby et al. Reported on a 15 year-old girl who rescued and also released a bat that had struck an internal window. The bat bit her left table of contents finger. The wound was washed v peroxide, but medical attention was no sought, and no rabies prophylaxis was administered. One month later she started to experience progressive neurological symptoms that were eventually diagnosed together rabies. The mainstay the her therapy was medically induced coma. Eight days later blood test demonstrated the she had begun to develop an immune response to the rabies virus. Eventually the coma was reversed, and also the patient progressively regained consciousness. She had actually severe neurological deficits, but gradually improved. She was discharged come her house after 76 days. 5 months after her initial hospitalization, she to be alert and communicative, however had persistent slurred speech and an unstable gait.

The report by Willoughby et al. Is an example of a case report – a detailed description of a single subject. The report is important since it demonstrates that it is feasible for victims of rabies come survive, even without post-exposure prophylaxis. However, we have actually no idea how efficient this treatment could be.

Case Series

A case collection is a report top top the characteristics of a group of topics who all have a particular disease or condition. Usual features amongst the group may imply hypotheses about disease causation. Note that the "series" may be small (as in the instance below) or it might be big (hundreds or thousands of "cases"). However, the cook limitation is the there is no to compare group. Consequently, usual features may imply hypotheses, but these need to be tested through some kind of analysis study before an association can be accepted as valid.

Example: Pneumocystis carinii pneumonia and also mucosal candidiasis in previously healthy homosexual men: proof of a brand-new acquired moving immunodeficiency.

Link to post by Gottlieb MS, et al: N Engl J Med 1981;305:1425-1431.


In 1980 –1981 4 previously healthy young guys were diagnosed through Pneumocystis carinii pneumonia, an unexplained "opportunistic" epidemic that had only been watched in immune compromised people with hereditary obstacle or in world with immune compromise as result of chemotherapy. The medical backgrounds didn"t suggest any kind of preexisting immunodeficiency, yet all had reduced immune responses and also low T cell counts. These unexplained infections said the possibility of a previously unknown disease. That was provided that all 4 men to be sexually energetic homosexuals, and in the case collection which was released in the new England newspaper of medicine the writer speculated that the immune dysfunction was as result of a sexually transmitted contagious agent.

This to be an extraordinarily essential case collection (a detailed description of qualities of a collection of civilization who all have actually the same disease) that argued that this brand-new syndrome was connected with sexual task in male homosexuals. Alerting the medical establishment and also proposing a hypothesis was an essential milestone in the AIDS epidemic, however, the association might not it is in securely established based upon this tiny case series. It to be not known how countless other individuals can be enduring from this brand-new syndrome. The was likewise not known what the ubiquity of homosexuality might be in others v this syndrome or just how this might compare come the as whole prevalence of homosexuality in the population that provided rise to the cases. As a result, this case series could not securely develop a precious association. Nevertheless, it laid the ground work-related for succeeding case-control studies and also cohort research studies (analytic studies) the did establish the risk components for this disease.

Example: oral Contraceptives and Hepatocellular Carcinoma?

There had actually been a number of case reports of liver cancers in young females taking oral contraceptives. A study was undertaken by contacting every one of the cancer registries collaborating v the American university of Surgeons. The investigators want to collect details on as countless of these rarely liver tumors as feasible across the US.

Table - oral Contraceptive Use among Women Who emerged Liver Cancer

Oral Contraceptive Use

Age 16-25 yrs.

Age 26-35 yrs.




Age 36-45 yrs.










What conclusions have the right to you draw from these data concerning a possible increased threat of liver cancer in mrs taking oral contraceptives? Think about it prior to you look in ~ the answer.


key Concept: The crucial to identifying a case series is that all of the subjects had in the study have actually the primary condition or outcome of interest. Because that example, an write-up reported ~ above 239 human being who obtained bird flu. The write-up might existing tables and also graphs that offered information about their age, occupation, wherein they lived, whether they live or died, etc., however basically the is a detailed description of the characteristics and outcomes in a group of human being who all had actually the same disease.

Video Summary: situation Reports and Case collection (6:59)

Cross-Sectional Surveys

Cross-sectional surveys evaluate the ubiquity of condition and the ubiquity of risk factors at the same point in time and provide a "snapshot" of diseases and also risk factors simultaneously in a characterized population. For example, US federal government agencies periodically send out large surveys to arbitrarily samples of the us population, asking about health status and also risk factors and behaviors at that point in time. The wellness Interview inspection (HIS) and also the national Health and also Nutrition Examination survey (NHANES) are great examples.


The health questionnaires you room asked to fill out as soon as you walk to a new physician or being processed for a brand-new job, or before entry into military organization are similar to cross-sectional surveys in that they ask about the health difficulties that you have (heart disease? diabetes? asthma?) and your present behaviors and risk determinants (e.g., exactly how old room you? carry out you smoke? What is her occupation?).

Cross-sectional surveys ask people their present status through respect to both exposures and diseases. This outcomes in two main disadvantages.

The temporal relationship in between exposure and condition outcomes can be unclear, i.e., which came first.Cross-sectional researches tend to recognize prevalent cases of lengthy duration, because people who die easily or recover quickly or who space no much longer employed in a details occupation are much less likely to it is in identified.

Consider the following instance in i beg your pardon a inspection was conducted amongst white male farm yard workers. The inspection asked numerous questions, but amongst them to be the questions: "Have you been told you have coronary heart condition (CHD)?" and "How would certainly you classify your level of physical activity?" The table below summarizes the findings.

Table - current Coronary Heart an illness Among Male farm Workers

# the Respondents

# Respondents

With CHD

Prevalence that CHD

per 1,000

Currently no Active

Currently physical Active







Note that the investigators did not follow these subjects over a duration of time, so they did not assess the "incidence" of heart disease. Instead, they asked the subjects inquiries designed to determine the pervasiveness of heart disease, i.e., the relationship of the study population that had actually heart an illness at this details point in time. When they divided the sample into physically active and inactive farmers and also computed the prevalence of heart an illness in each of these, they found that CHD was much an ext prevalent among the inactive farmers. However, this to be a cross-sectional examine that connected the pervasiveness of disease to the ubiquity of task at a suggest in time. Castle did not follow subjects gradually to monitor the advancement of heart disease (i.e., the incidence). Consequently, the temporal relationship in between the risk element of attention (physical inactivity) and the outcome (CHD) is unclear. Had actually the farmers to be physically active prior to emerging CHD? Or, walk they start to limit your physical activity after they arisen CHD? consequently physical inactivity can have been either a cause of heart disease, or it can have to be a consequence of CHD.

Large cross-sectional surveys are essential for monitoring wellness status and also health care needs the the populace over time, and they space sometimes helpful for suggesting possible associations between risk factors and diseases. However, the temporal relationship in between the risk factor and disease is typically unclear. Under this circumstances, they deserve to generate hypotheses, but these associations have to be experiment by proper analytical studies.

However, keep in mind that under part circumstances, the temporal partnership is clean on a cross-sectional survey. For example, if one performed a inspection of earnings of male and also female professors to watch if sex was associated with value inequities, we might regard this together an analytical study, since it is clear that sex was developed long before salary level. In this situation the temporal relationship between the "exposure" of attention (gender) and also outcome (salary paid) is clear; we recognize that sex was established prior to the salary was negotiated.So, in a feeling cross-sectional researches (and eco-friendly studies have the right to be believed of together an intermediate category between descriptive and analytic studies.

Video review on Cross-Sectional Surveys (8:25)


Ecological research studies (Correlational Studies)

These studies are differentiated by the fact that the unit of monitoring is not a person; fairly it is an entire populace or group. In essence, these researches examine the correlation in between the mean exposure in various populations with the as whole frequency of an illness within the populations.

In the study below investigators used business data to compute the overall consumption that meat by various nations. They then calculated the median (per capita) meat usage per human being by dividing full national meat usage by the variety of people in a given country. Over there is a clear linear trend; nations with the lowest meat intake have the lowest prices of colon cancer, and also the colon cancer rate amongst these countries progressively boosts as meat intake increases.


Note the in reality, people"s meat consumption probably differed widely within nations, and also the exposure the was calculated was an mean that assumes the everyone ate the median amount the meat. This typical exposure was then associated with the overall condition frequency in every country. The instance here says that the frequency of colon cancer increases as meat intake increases. The characteristic of environmental studies that is most striking is the there is no information around individual people. If the data to be summarized in a spread sheet, you would certainly not watch individual level data; you would see records with data on typical exposure in multiple groups.

Morgenstern note that, "Individual­ level variables space properties that individuals, and also ecologic variables are properties that groups. Come be more specific, ecologic measures might be classified into three types:

Aggregate actions are recaps (e.g. Means or proportions) that observations derived from people in each team (e.g. The ratio of smokers or median household income).Environmental steps are physical qualities of the location in i beg your pardon members of each team live or job-related (e.g. Air-pollution level or hrs of sunlight). Note that each environmental measure has an analogue in ~ the individual level, and also these individual exposures, or doses, typically vary among members of each group, despite they might remain unmeasured.Global actions are characteristics of teams or places for which over there is no distinct analogue at the separation, personal, instance level. Unlike accumulation and eco-friendly measures (e.g. Population density, level of social disorganization. Or the visibility of a details law).

Morgenstern goes on come note: "Ecologic research designs may be classified on 2 dimensions: (a) whether the primary group is measure up (exploratory vs analytic study); and also (b) even if it is subjects room grouped by ar (multiple-group study), by time (time-trend study), or through place and also time (mixed study). In spite of several practical advantages of ecologic studies, over there are countless methodologic difficulties that severely border causal inference, including ecologic and cross-level bias, troubles of confounder control, within-group misclassification, lack of adequate data, temporal ambiguity, collinearity, and also migration across groups."

For a detailed review that ecologic researches see monitor the connect to an article by Morgenstern H: Ecologic studies in Epidemiology: Concepts, Principles, and also Methods. Annual Review that Public health 1995;16:61-81.

To watch an extraordinary example of one ecologic study, play the video below developed by Hans Rosling. This is a magnificent example that examines the correlation between income and also life expectations in the countries of the civilization over time. The is likewise a wonderful example of a creative, engaging, and also powerful method to display a vast quantity the data.

Advantages of eco-friendly Studies:

The data required is typically readily available. Business data deserve to be offered to calculation a population"s full consumption of assets (possible risk factors) such as meat, tobacco, fish, etc. So, these researches are rapid & inexpensive.The "correlation coefficient" or one "r" value offers a measure up of how very closely the it was observed data point out conform come a straight line. Some authors say the the "r" worth is a measure up of the association in between the threat factor and the disease, but this is incorrect. The slope of the line would certainly be a measure of the strength of association. (See the food spreadsheet "Epi_Tools. XLSX" for a worksheet the calculates correlation coefficients). The worth of a correlation coefficient is native +1 (a perfect confident correlation) and also –1 (a perfect negative correlation). Check out the tabbed task below for examples.

Limitations of ecological Studies: that is important to be afflicted with in mind that the exposure in correlational research studies is the average exposure for an entire population or group. This outcomes in major limitations:

Since you don"t have any kind of information around the risk factor status or the outcome standing of separation, personal, instance people, girlfriend can"t directly attach the risk aspect to the disease, i.e., it is not clear that the civilization who ate the many meat were the persons who got colon cancer. This is sometimes referred to together "ecological bias" or the "ecological fallacy."Another limitation is the there is no effective means of taking into account, or adjusting for, other determinants that influence the result (confounding factors). As a result, an obvious correlation, or the lack of a correlation might be misleading. Because that example, one could find a strong correlation between the average number of hours that TV the town hall & the rate of coronary artery disease among different countries. However, this doesn"t necessarily median that TV per se is a risk element for CAD. There may be a variety of other differences in between the populations that are linked with higher rates of TV viewing: e.g., higher industrialization, less exercise, greater access of processed foods and saturated fat, and also so forth. And conversely, the absence of a correlation doesn"t necessarily imply that there is no association.Since the exposure levels represent mean exposure in a huge number that people, correlational studies deserve to mask more facility relationships, as shown below.

When a correlational study contrasted per capita alcohol intake to death rates native coronary heart an illness in different countries, it showed up that there was a fairly striking negative correlation.


However, a meta-analysis the prospective cohort studies which figured out mortality rates in topics for whom they had estimates of separation, personal, instance alcohol consumption, confirmed that there was actually a "J" shame relationship. The world who drank the most actually had the highest possible mortality rates; moderate drinkers had the lowest mortality. This connection was masking in the correlational study, because of the tiny percentage of human being who have an ext than 3 drinks every day.


Adapted from: Di Castelnuovo A, Costanzo S, et al.: Alcohol Dosing and Total Mortality in Men and also Women:

An updated Meta-analysis that 34 Prospective Studies. Arch Intern Med. 2006;166(22):2437-2445.

Video an overview for eco-friendly Studies (7:48)

Summary & Self-Check

Descriptive studies are helpful for:

Monitoring and reporting ~ above the health status and also health related habits in populationsIdentifying arising health problemsAlerting us to potential risks from bioterrorismEstablishing public wellness priorities for a populationEvaluating the performance of treatment programs andExploring potential associations in between "risk factors" and also health outcomes in stimulate to create hypotheses around the factors of disease.

See more: As I Rained Blows Upon Him, I Realized There Has To Be Another Way

Other ResourcesUniversity of phibìc Carolina (UNC) -Torok M and also Anderson M: "Focus on ar Epidemiology: Volume 5; worry 5:Introduction to Public health and wellness Surveillance."University of phibìc Carolina (UNC) - Anderson M: "Focus on ar Epidemiology: Volume 5; problem 6: Public wellness Surveillance Systems".Trifonov V, Khiabanian H, Rabadan R: geographic Dependence, Surveillance, and Origins of the 2009 Influenza A (H1N1) Virus. Perspective post in: N. Engl. J. Med. 2009;361(2):115-119. Scallan E, Hoekstra RM, Angulo FJ, et al. Foodborne illness Acquired in the United claims - significant Pathogens. Emerging infectious Diseases 2011;17(1):7-15. Marsden-Haug N, Foster VB, Gould PL, Elbert E, Wang H, Pavlin JA. Code-based syndromic surveillance for influenzalike disease by International group of Diseases, 9th revision. Emerg epidemic Dis, Feb. 2007;13(2):207-216.