  There is effect modification if the magnitude or stability of a treatment
effect varies systematically with the level of an observed covariate. A larger
or more stable treatment effect is typically less sensitive to bias from
unmeasured covariates, so it is important to recognize effect modification when
it is present. We illustrate a recent proposal for conducting a sensitivity
analysis that empirically discovers effect modification by exploratory methods,
but controls the family-wise error rate in discovered groups. The example
concerns a study of mortality and use of the intensive care unit in 23,715
matched pairs of two Medicare patients, one of whom underwent surgery at a
hospital identified for superior nursing, the other at a conventional hospital.
The pairs were matched exactly for 130 four-digit ICD-9 surgical procedure
codes and balanced 172 observed covariates. The pairs were then split into five
groups of pairs by CART in its effort to locate effect modification. The
evidence of a beneficial effect of magnet hospitals on mortality is least
sensitive to unmeasured biases in a large group of patients undergoing rather
serious surgical procedures, but in the absence of other life-threatening
conditions, such as a comorbidity of congestive heart failure or an emergency
admission leading to surgery.
