st: sts graph, na yscale(log) vs stphplot

["JAC Sterne, Social Medicine" <Jonathan.Sterne@bristol.ac.uk>]

Dear statalist

Stata provides two graphical methods to assess the proportional hazards 
assumption (1) graphing the cumulative hazards on a log scale using -sts 
graph, na yscale(log) or (2) using the stphplot command. Although the point 
estimates are derived in different ways these two methods are equivalent 
because log(cumulative hazard) is the same as log(-log(survival).

I prefer (1), because it is easier to understand why graphs of 
log(cumulative hazard) should be parallel if the proportional hazards 
function is true. However, there seems to be a problem with the graph 
produced by sts graph, na yscale(log) in Stata 8. Because the cumulative 
hazard (by definition) starts at 0, the graph gets squeezed up to the top 
unless you start plotting beyond the time of the first event using the 
tmin() option. And when comparing cumulative hazards in several groups this 
means starting  beyond the minimum time in all groups, which if one of the 
groups is small may mean omitting much of the data.

Have I (a) misunderstood, or (b) missed an easy workaround? Or is this a 
bug/feature requiring improvement?

Thanks

Jonathan

----------------------

Jonathan Sterne
Department of Social Medicine
University of Bristol
Canynge Hall, Whiteladies Road
Bristol BS8 2PR

Phone  0117 928 7396
Fax    0117 928 7325
E-mail jonathan.sterne@bristol.ac.uk
web    www.epi.bris.ac.uk


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