@article{ref1,
title="The effect of missing values for covariates",
journal="Epidemiology",
year="2002",
author="Whitlock, G. and Clark, Tony",
volume="13",
number="1",
pages="113-113",
abstract="Abstract unavailable.",
language="",
issn="1044-3983",
doi="",
url="http://dx.doi.org/"
}