Study population

Study population investigated:

• when using evidence from clinical studies to inform treatment decisions involves accepting the 'average' results obtained in a group of people and generalising these to individual patients. However clinical studies often employ strict subject inclusion and exclusion criteria - though this approach can help to remove some potential biases in the study
  • it may mean that the characteristics of subjects in the study do not reflect the characteristics of all patients who will receive the intervention
    • possible reasons why study subjects may not reflect patients seen in routine clinical practice include
      • different risk factors for disease
      • presence of co-morbidities
      • differing levels of care received in different care settings
      • these factors, and many potential others, can mean that responses or susceptibilities to the effects of interventions may differ between those observed in study subjects and those potentially seen in actual patients

Notes:

• allocation by chance in a randomised controlled trial should mean that the groups being compared are similar in terms of both measured and unmeasured baseline factors - however this is not so in cohort studies, and therefore cohort studies are vulnerable to selection bias (2). Therefore consider possible selection bias and confouding factors when appraising a study

Reference:

1. MeReC Briefing (2005);30:1-7.
2. Rochon PA et al.Reader's guide to critical appraisal of cohort studies: 1. Role and design. BMJ 2005;330:895-7.