A correlated
variable may still contribute predictive information. The correct
answer is (c), correlated variables may well be combined to
obtain a more stable measurement of the underlying biological
phenomenon. An example is taking the mean of diastolic and systolic
blood pressure. Related variables may also be combined in a
summary variable. For example, an atherosclerotic risk variable
may be created based on the presence of various signs that indicate
atherosclerosis (Krijnen
et al., 1998). Also, we might count the number of signs
to form such a variable (Harrell
et al., 1985).