  The Cox proportional hazards model is routinely used to analyze time-to-event
data. To use this model requires the definition of a unique well-defined time
scale. Most often, observation time is used as the time scale for both clinical
and observational studies. Recently after a suggestion that it may be a more
appropriate scale, chronological age has begun to appear as the time scale used
in some reports. There appears to be no general consensus about which time
scale is appropriate for any given analysis. It has been suggested that if the
baseline hazard is exponential or if the age-at-entry is independent of
covariates used in the model, then the two time scales provide similar results.
In this report we provide an empirical examination of the results using the two
different time scales using a large collection of data sets to examine the
relationship between systolic blood pressure and coronary heart disease death
(CHD death). We demonstrate, in this empirical example that the two time-scales
can lead to differing results even when these two conditions appear to hold.
