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).

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.

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...

I think you have it backwards. The risk of the individual surgeries is not increased. Only the risk that a surgeon will be confronted with an above average critical surgery is increased.

It is not even clear that there actually is a problem. It's just a weird way to slice the data to produce an effect.

> The risk of the individual surgeries is not increased.

They never said that it did (unless they've edited their comment since you replied to it). They just said that the percentage [of deaths on surgeons' birthdays] will increase, and that is correct.

I do not completely understand your points.

Shouldn't hospitals have multiple surgeons?

If one of them is on vacation, the other one does the work, and vice versa?

But the more you avoid birthday surgeries altogether, the more those that end up still happening are extreme cases with high mortality rates. So if your target is to avoid this scary statistical anomaly, you might instead want to promote more benign surgeries on birthdays.

The types of surgeries performed should be controlled for when performing a statistical study.

They were. This entire line of criticism comes from not reading the study methods.

I don't think surgeons are fongible assets. There is a lot of planning and study taking place before a complex operation. Not to mention most surgeons have specialties.

The long term plans and studied for surgeries can be moved one day forward or backward basically by definition.

Except if something changes suddenly and surgery is suddenly needed - that’s what makes it an emergency.

A surgeon seems more fungible than a developer, to me.

All sorts of standards, compliance, and licensing requirements.

Surgeons are generally specialists in one particular area and possibly even specialists within that area (ie: only do knee replacements, etc.). Surgeons get better with experience, it's a skill, and different surgeries are different enough that experience doesn't transfer too much. There's also changes over time in best practices so skills degrade not just due to lack of recent experience. If I remember the best predictor of the outcome of your surgery is how many similar surgeries the doctor does per year.

Nobody's making any points about developers

My gut instinct is that this is incorrect, too, but I don't know enough about surgery to make a compelling argument.

I'd back myself to pick up Ruby (a language I've never touched before) and be productive, more than I'd trust a surgeon who only has experience with heart surgery to operate on my brain. Maybe that's ignorant of me.

Even if a Ruby bug would mean death?

EDIT: I take this back. I missed the point of the argument.

That'd change the risks, sure.

I don't think that's the scenario @jrh206 was talking about, though. Most code written in Ruby doesn't have the sort of immediate risk to life or limb surgeries do.

Those requirements are fairly general.

They don't test for "I've been doing this particularly tricky type of bone biopsy right next to the spinal cord for decades" scenarios.

For non-emergency surgeries it's often a long-term relationship where the same doctor who has seen the patient a few times would be the one operating - so if that particular surgeon isn't available for whatever reason, the planned operation would be rescheduled to a different date with the same doctor, not to a different doctor in the same day.

There’s research, consultations, who is covered under what insurance, etc.

Yes, it does really mean you need to dig into actual causation or the law of unintended consequences will bite you in the ass.