@article{ref1,
title="Statistical and design issues in studies of groups",
journal="Injury prevention",
year="2002",
author="Cummings, P. and Koepsell, Thomas D.",
volume="8",
number="1",
pages="6-7",
abstract="<p>[Abstract unavailable]</p> <p>Language: en</p>",
language="en",
issn="1353-8047",
doi="",
url="http://dx.doi.org/"
}