Bias reduction in conditional logistic regression

We employ a general bias preventive approach developed by Firth ( Biometrika 1993; 80 :27–38) to reduce the bias of an estimator of the log‐odds ratio parameter in a matched case–control study by solving a modified score equation. We also propose a method to calculate the standard error of the resultant estimator. A closed‐form expression for the estimator of the log‐odds ratio parameter is derived in the case of a dichotomous exposure variable. Finite sample properties of the estimator are investigated via a simulation study. Finally, we apply the method to analyze a matched case–control data from a low birthweight study. Copyright © 2010 John Wiley & Sons, Ltd.