Four types of bias threatening internal validity.

Abstract

<p>Adapted from <a href="http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.1000245#pmed.1000245-Juni1" target="_blank">[12]</a>,<a href="http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.1000245#pmed.1000245-Altman1" target="_blank">[13]</a>.</p