TY  - JOUR
T1  - COronary heart disease risk factors and mortality
AU  - Banack HR, Harper S, Kaufman JS
Y1  - 2012/03/21
N1  - 10.1001/jama.2012.324
JO  - JAMA
SP  - 1137
EP  - 1138
VL  - 307
IS  - 11
N2  - 
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).

SN  - 0098-7484
M3  - doi: 10.1001/jama.2012.324
UR  - http://dx.doi.org/10.1001/jama.2012.324
ER  -