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).