RT Journal
A1 Sharkansky S
T1 SChool-based health center use and high school dropout rates
JF Archives of Pediatrics & Adolescent Medicine
JO Archives of Pediatrics & Adolescent Medicine
YR 2012
FD July 1
VO 166
IS 7
SP 675
OP 677
DO 10.1001/archpediatrics.2012.550
UL http://dx.doi.org/10.1001/archpediatrics.2012.550
AB 
The apparent association between lower dropout rates and SBHC visits might have been exaggerated by their methods, which seem to have introduced time-dependent (or “immortal time”) bias. This bias, apparently common in clinical studies, arises from modeling a time-dependent treatment in a hazards regression as a fixed baseline covariate instead of as a time-varying covariate.2 In this case, Kerns et al ignored the timing of SBHC visits, modeling the treatment as a fixed covariate representing the frequency of the subject's SBHC visits, averaged over his or her entire high school career.