Addressing the unit of analysis in medical care studies: A systematic review

Aaron W. Calhoun, Gordon H. Guyatt, Michael D. Cabana, Downing Lu, David A. Turner, Stacey Valentine, Adrienne G. Randolph

Research output: Contribution to journalReview articlepeer-review

14 Scopus citations


OBJECTIVE: We assessed the frequency that patients are incorrectly used as the unit of analysis among studies of physicians' patient care behavior in articles published in high impact journals. METHODS: We surveyed 30 high-impact journals across 6 medical fields for articles susceptible to unit of analysis errors published from 1994 to 2005. Three reviewers independently abstracted articles using previously published criteria to determine the presence of analytic errors. RESULTS: One hundred fourteen susceptible articles were found published in 15 journals, 4 journals published the majority (71 of 114 or 62.3%) of studies, 40 were intervention studies, and 74 were noninterventional studies. The unit of analysis error was present in 19 (48%) of the intervention studies and 31 (42%) of the noninterventional studies (overall error rate 44%). The frequency of the error decreased between 1994-1999 (N = 38; 65% error) and 2000-2005 (N = 76; 33% error) (P = 0.001). CONCLUSIONS: Although the frequency of the error in published studies is decreasing, further improvement remains desirable.

Original languageEnglish (US)
Pages (from-to)635-643
Number of pages9
JournalMedical Care
Issue number6
StatePublished - Jun 2008
Externally publishedYes


  • Cluster analysis
  • Interpretation of clinical trials
  • Statistical bias
  • Study design
  • Unit of analysis error

ASJC Scopus subject areas

  • Public Health, Environmental and Occupational Health


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