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Biometrika Advance Access originally published online on September 25, 2008
Biometrika 2008 95(4):992-996; doi:10.1093/biomet/asn039
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© 2008 Biometrika Trust

Miscellanea

On the consequences of overstratification

B. L. De Stavola

London School of Hygiene and Tropical Medicine, London WC1E 7HT, U.K. bianca.destavola{at}lshtm.ac.uk

D. R. Cox

Nuffield College, Oxford OX1 1NF, U.K. david.cox{at}nuffield.ox.ac.uk

Received for publication 1 April 2008. Revision received 1 July 2008.
  Abstract

It is common, in particular in observational studies in epidemiology, to impose stratification to adjust for possible effects of age and other variables on the binary outcome of interest. Overstratification may lower the precision of the estimated effects of interest. Understratification risks bias. These issues are studied analytically. Asymptotic results show that loss of efficiency depends on the true effect and on a measure of the average imbalance across strata between exposed and unexposed individuals. Bias depends on the correlation between stratum-specific size imbalances and event rates in the unexposed. Approximate results are also given. An example is used.

Key Words: Bias • Conditional likelihood • Confounder • Epidemiology • Information • Logistic model: Matched pairs • Poisson distribution


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