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When generating survival times to simulate Cox proportional hazards models, does it matter to generate them in days or in years?

In theory, I guess it does not matter. But in practice? Is there a preference regarding computational issues?

Thank you! Marco

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    $\begingroup$ There are no computational issues. Choose a time unit that is readily interpretable. $\endgroup$ – whuber Feb 11 '11 at 15:15
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    $\begingroup$ I believe it's advantageous to avoid ties when dealing with time as a continuous variable. Therefore I think you'd want to base your results on the finest-grained measurements you can get (if that is part of what your question implies). $\endgroup$ – rolando2 Feb 11 '11 at 18:53
  • $\begingroup$ @Whuber: Ok Ok, thanks! @rolanda2: Yes, you are right. But if I generate survival times in years I keep all decimal points. Thx $\endgroup$ – ocram Feb 12 '11 at 7:08
  • $\begingroup$ @whuber, @rolando2 Make answers of that. $\endgroup$ – user88 Feb 13 '11 at 14:36
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There's no computational issues behind using a different time scale. I would make sure the time scale you used is biologically/practically meaningful and plausible to collect. If you're talking about the time-to-failure of a single node in a computer cluster, you might be able to say you'd know the event time down to a fraction of a second. If you're talking about time until HIV seroconversion, you'd be lucky to know the day, and more likely know the week or month. Simulation studies are most useful when they're conducted similarly.

I wouldn't worry to much about ties in your time - they'll happen in real-life too, and its useful for your situation to know how to handle them.

So I'd say, generally, pick a good, interpretable time scale, and retain as much precision as is reasonable in a real-world setting.

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