1. A Brief Introduction to Epidemiology - VIII (Epidemiologic Research Designs: Cross-Sectional/Prevalence Studies)
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Betty C. Jung, RN, MPH, CHES

2. Learning/Performance Objectives
To develop an understanding of:
What cross-sectional studies are
The value of such studies
The basic methodology
Pros and cons of such studies
This lecture seeks to provide you with a basic understanding of Cross-Sectional Studies, the most common observational study used by Epidemiologists.
Additional textbook resources can be found in the following annotated bibliographies on my Web site:
<UL>
<LI><B><A HREF= “ Annotated Biostatistics Bibliography </A></B></LI>
<LI><B><A HREF=
“ Epidemiology Bibliography </A></B></LI>
<LI><B><A HREF=“ Research Methods</A></B></LI>
<LI><B><A HREF=“ Research Practice (A - L)</A></B></LI> <LI><B><A HREF=“ Research Practice (M - Z)</A></B></LI>
<LI><B><A HREF=“ Statistics Bibliography </A></B></LI></LI></UL>
Sources for this lecture include: L, Gordis (1996) Epidemiology, R.M. Page, G.E. Cole & T.C. Timmreck (1986) Basic Epidemiological Methods and Biostatistics, and W.H.O. (1990) Basic Epidemiology.

3. Introduction
Epidemiology studies the distribution of disease in a number of ways.
The two major categories of epidemiological studies are: Observational and experimental studies.
Most epidemiological studies are observational.
While Epidemiology is the study of the distribution of disease, the research methods used and developed by epidemiologists have been used also to study “disease-related” issues. For example, health services researchers look at how health services differ for different populations. Health education researchers look at how people react to disease and how these behaviors impact quality of life.
Thus, findings from epidemiological studies should serve as the foundation for research in health education, health policy, etc. because a better understanding of how disease impacts populations is crucial before we can even think of how to better deliver services, educate people about health behaviors, and develop rational public policy.

4. Epidemiological Study Designs
Observational Studies - examine associations between risk factors and outcomes (Analytical - determinants and risk of disease, and descriptive - patterns and frequency of disease)
Intervention Studies - explore the association between interventions and outcomes. (Experimental studies or clinical trials)
Similar to how Research is viewed in general, there is no clear typology to epidemiological study designs. This has made it difficult to explain these studies in a simple way.
It can be said, in general, that there are two main types of studies.
Observational studies are those in which the researchers do not interfere or manipulate any of the factors under study. They record their observations of what’s going on, and explain what they observe with measures of associations.
While we would like to say that A causes B, in order for these kinds of studies to attempt such an explanation, there would have to be many, many studies that observed the same chain of events before there is a consensus that A causes B, if that.
Interventional studies are when researchers attempt to show that A causes B by actually manipulating those factors they think have something to do with causing some outcome. These usually look at how well some treatments work.
In Public Health, it is difficult to show that a specific factor caused a particular outcome, when we are talking about populations. There are usually many factors that can result in a particular outcome. And because it is unethical to conduct testing on human subjects without people’s consent, conducting research involving general populations is infeasible as well. So, from a feasibility standpoint, observational studies are more common, and if done well, can provide us with insights on how to control disease and improve quality of life.

5. Research Designs in Analytic Epidemiology
Ecologic Designs Cross-Sectional Study
Case-Control Study
Cohort Study
The more common study designs in Epidemiology include:
Cohort Study - gold standard when exploring the association between a risk factor or participation in a health program, and health status outcomes or health events.
Case-Control Study - more commonly used, less expensive, when outcome of interest is rare, and a random sample from population would yield too few cases. Involves a retrospective assessment of differential exposure between 2 comparison groups.
Data from Entire Population - considered cross-sectional since collection or sampling of the exposure and outcome information is at the same point in time. Considered prevalence data. A relationship between exposure and outcome is the ratio of 2 prevalence rates, or relative prevalence.
Ecologic Designs - Data for exposure and disease are used in aggregated form and cannot be linked at the individual level.

6. Cross-Sectional Studies
“Snapshot Studies” (Paffenbarger, 1988)
Observations at a single hypothetical point in time
Each subject assessed once at point in time.
Point Prevalence Studies
Cross-Sectional studies are examples of applied research. Applied research is probably THE research approach taken by Public Health Practitioners in the course of their work. Because Public Health seeks to ensure the health of the Public, it does this by first trying to prevent problems before they occur. This is what Prevention is all about. And, if a problem has already occurred, Public Health Practitioners work hard to control the situation. If it affects a lot of people, and public health interventions, strategies, programs can address the problem, then surveillance systems will be developed and maintained. These systems help to keep the problem under control, by monitoring the problem as well as providing data to evaluate the effectiveness of the solutions (interventions, strategies, programs). These strategies seek to prevent the problem from occurring again.
In fact, Public Health has been so successful in ensuring the Public’s health that it is sometimes taken for granted, until some disaster occurs. As a result, public health programs don’t always get the funding they can really use to remain vigilant. These issues are ones of Public Health Infrastructure, which Healthy People 2010 (US Public Health Service planning document) does address. So, in order to conduct applied research in Public Health, I think it is essential that there is an infrastructure that supports this type of research so Public Health can fulfill its mission to ensure the health of the Public.

7. Examples
National Health & Nutrition Examination Surveys - allow for current estimates of prevalence for health parameters
Youth Risk Behavior Surveys - prevalence of prior health-risk behaviors
Cross-Sectional studies are useful in giving you a “big picture” look at a problem. It seeks to describe those who are affected by the disease or issue studied. Such national surveys that are consistently conducted over time help policymakers and program planners to better assess how much money and effort should be devoted to a particular problem. And, in some ways, such studies also monitor the progress being made in addressing the problem.

8. Value Observe high prevalence of risk factors in those with disease
Suggests possibility of certain risk factors as cause of common disease
Quantify with point prevalence rates
Cross-Sectional studies are useful in looking at health problems and issues that affect a large portion of the population, and at the same time help to identify those factors that contribute to the problem or issue. Once the factors are identified, then these factors can be targeted for interventions.

9. Value
Cannot tell why any health differential exist, but that they do exist
Good for generating hypotheses about the cause of disease
Can be used to initiate and evaluate effective health services programs
Most commonly used strategy

10. Pros One time exam/interview Good for program and service planning
To determine associations between variables and generate hypotheses
Description of relative distribution, directs case finding

11. Cons Cannot predict future health event happenings
Limited usefulness for outbreak and epidemics
Does not allow to determine which came first
Only describes what exist at time of study

12. References
For Internet Resources on the topics covered in this lecture, check out my Web site: