[FabHK]

You could control for patient characteristics (age, severity of the condition, etc), and that was indeed done here, see the paper. It also specifically addresses this issue:

> The major threat to the internal validity of our findings is that surgeons may selectively operate on sicker and more complex patients on their birthday, perhaps because those patients cannot have their procedures delayed. However, this is unlikely to explain our findings because we found that patients who underwent surgery on the surgeon’s birthday were similar in all observable characteristics to patients who underwent surgery on other days. Furthermore, severity of illness as measured by predicted mortality, and the number of procedures performed per surgeon, also did not differ based on whether a surgery occurred on a surgeon’s birthday compared with other days.

It seems to me that the analysis is quite carefully done:

> Findings were qualitatively unaffected by: using in-hospital mortality instead of 30 day mortality; additionally adjusting for the timing of the surgery; including both hospital and surgeon fixed effects in the same regression models; excluding potentially outlier surgeons with the highest mortality; using logistic regression models instead of linear probability models: using random effects models instead of fixed effects models; restricting our analysis to surgeons who performed procedures on their birthdays; additionally adjusting for the day of the year; or excluding surgeons who were born on the outlier birthdays (supplementary eTables 5-13). [...] The study findings were qualitatively unaffected when the analysis was restricted to procedures with the highest average mortality or to patients with the highest severity of illness (supplementary eTables 16 and 17).

[nkurz]

This is a great point to raise, but it's worth noting that it directly contradicts the GP's anecdotal observation. Had they instead found greater severity on birthdays and attempted to statistically correct for it, the two would be compatible. Instead, they looked, and found that there was no underlying difference to correct for. The question then becomes whether the GP is wrong, whether the hospital in question didn't have such a policy, or whether the measurements used in the study were insufficient to pick up the difference in severity.

I don't know which of these is true, but despite the apparent statistical significance of the finding, I wouldn't be confident assuming that the result is generally applicable. While not impossible, it strikes me as suspicious that they found no differences whatsoever in the surgeons' birthday vs non-birthday schedules. I somewhat wonder if by "no difference" they really meant "no statistically significant difference", which in this case wouldn't justify their lack of adjustment.

[waterhouse]

Furthermore, note that there really is a significant "avoiding surgeries on birthdays" effect: 2064 in 980,876 operations were done on a birthday, which is 1 in 475, rather than the 1 in 365 if there were no such effect. That's a reduction of 23%, which is rather suspicious given that we're trying to explain a 23% increase in mortality rate.

So what mechanism is responsible for that reduction, and is it likely to affect surgeries differently based on how urgent and specialized (and therefore dangerous) they are? Since the authors restricted it to surgeons that have done at least one surgery on their birthday, that rules out blanket "never on birthday" policies. It seems like the only mechanism that wouldn't affect them differently is "the surgeon is already on vacation in another country and can't get here for the operation" (and they choose to take vacations on their birthday more frequently). One could probably check vacation-day records relatively easily...