Volume and Outcome

To the Editor: The attempt of Birkmeyer et al. (April 11 issue) 1 to correlate low procedure-specific hospital volume with increased mortality has methodologic and interpretive problems. The investigators used data from the Medicare Provider Analysis and Review (excluding those for Medicare patients enrolled in health maintenance organizations) and from the Nationwide Inpatient Sample, without verification, to estimate procedure-specific hospital volume. The correlation between Medicare volume and hospital volume (correlation coefficient, 0.97) probably resulted from mathematic coupling, 2 , 3 which occurs when variables are shared. Were the relations between outcome and volume significant when only Medicare volumes were analyzed? No proof of . . .