A SAS package for logistic two-phase studies

Two-phase designs,in which for a large study a dichotomous outcome and partial or proxy information on risk factors is available,whereas precise or complete measurements on covariates have been obtained only in a stratified sub-sample,extend the standard case-control design and have been proven useful in practice. the application of two-phase designs,however,seems to be hampered by the lack of appropriate,easy-to-use software. this paper introduces sas-twophase-package,a collection of sas-macros,to fulfill this task. sas-twophase-package implements weighted likelihood,pseudo likelihood and semi-parametric maximum likelihood estimation via the em algorithm and via profile likelihood in two-phase settings with dichotomous outcome and a given stratification.