Fifth, the analysis presented here is limited to two time points and may not represent a trend. Sixth, it is not possible to completely rule out the possibility of a systematic or methodological bias in our results. However, as is stated throughout this text, most NHBS procedures remained unchanged from 2008 to 2011 and those that did change do not appear to have resulted in different samples. Consequently, a methodological bias is unlikely. Finally, the analysis presented here did not include risk behaviors.