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Chapter 12. Observational Study Designs

Bradley G. Hammill

Observational studies in clinical research can be classified as either analytic or descriptive (Table 12–1). Analytic observational studies are similar to randomized, controlled clinical trials in that the goal is to estimate the causal effect of an exposure on an outcome. Also similar to trials, analytic observational studies always include some type of comparison group, against which the experience of the exposed group is compared. Well-designed analytic studies can generate strong evidence for or against a stated hypothesis. Descriptive studies, on the other hand, aim to describe the characteristics or experiences of a particular patient group. Even well-designed descriptive studies cannot be used to draw strong conclusions about the effect of an exposure on an outcome. Instead, these studies are often used to generate study questions that can then be tested by more rigorous methods.

Table 12–1 Comparison of Features of Analytic and Descriptive Study Designs
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Table 12–1 Comparison of Features of Analytic and Descriptive Study Designs

Analytic designs

Descriptive designs

Case-control study

Cohort study

Prevalence study

Ecologic study

Case report/case series

Study purpose

Hypothesis testing

Hypothesis testing

Hypothesis generating

Hypothesis generating

Hypothesis generating

Primary study characteristics

Selection of study population by outcome status

Need to ascertain prior exposures

Selection of study population by exposure status

Need to ascertain subsequent outcomes

Single point-in-time survey of exposures and outcomes

Exposure and outcome data are aggregate measures

Detailed information regarding one or multiple interesting medical cases

Typical study population size

Small

Large

Can be small or large

(Not person level)

Very small

Temporal relationship between exposure and outcome

Difficult to establish

Easier to establish

Unknown

Difficult to establish

Usually established

Best use of study design

When outcome is rare

When exposure is rare

To establish prevalence of a disease in the sampled population

When it is infeasible to measure exposure at the individual level (e.g., environmental exposures)

To report initial results or potential safety signals for a new therapy or procedure

Primary challenge

Selection of appropriate controls

Avoiding differential recall bias when assessing exposure

For prospective designs, following study participants over time and waiting for events to occur

Not usually possible to discern timing of exposures and outcomes

No assurance that exposed persons are the same as those who experienced the outcome

No control group for comparison

Very limited population size

Although many observational study designs are available to researchers (1), a few are most widely used and will be described below. The analytic study designs presented are the case-control study and the cohort study. The descriptive study designs presented are the ecologic study, the cross-sectional prevalence survey, and case reports or case series.

Case-Control Studies

In a case-control study, the study population is selected based on a person's outcome status (2). For most case-control studies, the outcome is a disease. Cases are those that have or have had the ...

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