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


From   "JAC Sterne, Social Medicine" <[email protected]>
To   [email protected]
Subject   st: sts graph, na yscale(log) vs stphplot
Date   Tue, 27 May 2003 13:07:53 +0100

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 [email protected]
web www.epi.bris.ac.uk


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