Invited commentary: positivity in practice

Am J Epidemiol. 2010 Mar 15;171(6):674-7; discussion 678-81. doi: 10.1093/aje/kwp436. Epub 2010 Feb 5.


Positivity, or the experimental treatment assignment assumption, requires that there be both exposed and unexposed participants at every combination of the values of the observed confounders in the population under study. Positivity is essential for inference but is often overlooked in practice by epidemiologists. This issue of the Journal includes 2 articles featuring discussions related to positivity. Here the authors define positivity, distinguish between deterministic and random positivity, and discuss the 2 relevant papers in this issue. In addition, the commentators illustrate positivity in simple 2 x 2 tables, as well as detail some ways in which epidemiologists may examine their data for nonpositivity and deal with violations of positivity in practice.

Publication types

  • Research Support, N.I.H., Extramural
  • Comment

MeSH terms

  • Bias
  • Biometry / methods
  • Causality*
  • Confounding Factors, Epidemiologic*
  • Data Interpretation, Statistical*
  • Epidemiologic Methods
  • Humans
  • Propensity Score