I am running an analysis in a large population registry where individuals enter and leave the dataset at different time points.

When i estimated the IRR between two distinct groups and a health outcome, and then a HR, I am finding a small difference (IRR 2.18, HR 2.01).

Shouldn't they have been identical, since I am not adjusting for any covariates, etc? Are there any reasons that could explain why they are not identical?

Any thoughts would be appreciated.


Less is more. Stay pure. Stay poor.

Incidence Relative Rate
Hazard Ratio

If so, I would imagine not, since they are different models and likely use different distributions and HR addresses time to event. So I think even if follow-up was equal, they may still differ.