Lesson 7: The Study Cohort

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In a multi-center trial, even when study eligibility criteria are carefully described and followed precisely by different investigators at different locations, there can be enough patient heterogeneity and differences in protocol interpretation that the results can vary greatly across institutions. Thus, the differences in results actually could be due to different selection factors at the different institutions. Recruitment strategies might be different. Due to this, different patients are recruited into the study.

Eligibility criteria also define the accrual rate for a trial. Although tighter eligibility criteria lead to a more homogeneous trial, they yield a slower accrual rate. It might be more difficult to meet all of the criteria you specify using strict eligibility criteria.

Learning objectives & outcomes

Upon completion of this lesson, you should be able to do the following:

  • Compare the benefits and limitations of narrowly defined eligibility criteria to broadly defined eligibility criteria
  • Recognize the impact of the 'healthy worker effect'.
  • Write inclusion and exclusion criteria that are less subject to misinterpretation.
  • Consider the advantages and disadvantages of a run-in period or extended baseline for a study.
  • Use a simple method to monitor patient accrual.
  • Recognize barriers to patient participation in clinical trials
  • Distinguish between an efficacy study and an effectiveness trial.


Gotay, CC. (1991). Accrual to cancer clinical trials: directions from the research literature. Soc. Sci. Med. 33: 569-577.

Piantadosi Steven. (2005) The study cohort, Treatment allocation. In: Piantadosi Steven. Clinical Trials: A Methodologic Perspective. 2nd ed. Hobaken, NJ: John Wiley and Sons, Inc.