Sounds like you are making excellent progress. Great.
Piers likely hit the nail on the head. Given the large N, it is very
very likely an observational study confounded by selection bias.
Very difficult to really adjust for all confounders in that situation.
I doubt a randomized trial would ever be done, however.
Mike
ps: for reasons not understood, people who have bilateral replacment
tend to live longer and by a statistically significant margin.
Chuck Norcutt
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