RT Journal
A1 Banack HR, Harper S, Kaufman JS
T1 COronary heart disease risk factors and mortality
JF JAMA
JO JAMA
YR 2012
FD March 21
VO 307
IS 11
SP 1137
OP 1138
DO 10.1001/jama.2012.324
UL http://dx.doi.org/10.1001/jama.2012.324
AB 
To the Editor: The study by Dr Canto and colleagues1 provides an example of an epidemiological phenomenon that deserves wider recognition. Differential selection from an underlying population cohort into a study data set can reverse the direction of observed associations, making a deleterious factor appear protective. It is well-known that conditioning on a variable that is affected by both an exposure and outcome can produce a distortion known as selection bias.2 Admission into the analysis data set in this study was a function of both the exposure (number of cardiovascular risk factors) and the outcome (all-cause, in-hospital, or 30-day mortality) because deaths occurring before hospitalization and patients with existing cardiovascular disease diagnoses were excluded. Approximately 30% of MIs lead to death prior to hospitalization.3 In the study, 75% of those who were admitted to the hospital after their MI were excluded (1.62 million of 2.16 million; Figure 1 in the article).