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Evidence-Based Medicine: Study Objective, Direction & Timing

Study Objectives

  • Analytic (Explanatory) Study: The objective of an analytic study is to make causal inferences about the nature of hypothesized relationships between risk factors and outcomes. Statistical procedures are used to determine if a relationship was likely to have occurred by chance alone. Analytic studies usually compare two or more groups, such as case-control studies, cohort studies, randomized controlled clinical trials, and laboratory studies.
  • Descriptive Study: The objective of a descriptive study is to describe the distribution of variables in a group. Statistics serve only to describe the precision of those measurements or to make statistical inferences about the values in the population from which the sample was taken.


  • Prospective Study (Data): Data collection and the events of interest occur after individuals are enrolled (e.g. clinical trials and cohort studies). This prospective collection enables the use of more solid, consistent criteria and avoids the potential biases of retrospective recall. Prospective studies are limited to those conditions that occur relatively frequently and to studies with relatively short follow-up periods so that sufficient numbers of eligible individuals can be enrolled and followed within a reasonable period.
  • Retrospective Study (Data): All events of interest have already occurred and data are generated from historical records (secondary data) and from recall (which may result in the presence of significant recall bias). Retrospective data is relatively inexpensive compared to prospective studies because of the use of available information and is typically used in case-control studies. Retrospective studies of rare conditions are much more efficient than prospective studies because individuals experiencing the rare outcome can be found in patient records rather than following a large number of individuals to find a few cases.


  • Contemporary (Concurrent) Comparison: Comparison is between two groups experiencing the risk factor or the treatment at the same time. Contemporary comparison has the major advantages that symmetry of unknown risk factors for the condition that change over time is maintained and that measurement procedures can be performed as similarly as possible on both groups.
  • Historical (Non-concurrent) Comparison: Comparison is of the same group or between groups at different times that are not experiencing the risk factor or the treatment at the same time. Historical comparison is often used to allow a group to serve as its own historical control or is done implicitly when a group is compared to expected standards of performance. This design provides weak evidence because symmetry isn’t assured. It is very susceptible to bias by changes over time in uncontrollable, confounding risk factors, such as differences in climate, management practices and nutrition. Bias due to differences in measuring procedures over time may also account for observed differences.

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