[throwawaylolx]

Why are you and others assuming this was the case and it wasn't the original hypothesis they had in mind?

They explain the reasoning for selecting this hypothesis to test:

>Operations performed on birthdays of surgeons might provide a unique opportunity to assess the relationship between personal distractions and patient outcomes, under the hypothesis that surgeons may be more likely to become distracted or feel rushed to finish procedures on their birthdays, and therefore patient outcomes might worsen on those days.

[watwut]

Because popular hobby on HN is trying to find mistakes in studies by guessing from title without reading article.

Incidentally, interestingly, that is how bias in real world often works - people making different assumptions for different groups in absence of evidence.

[andrewla]

Did they pre-register before conducting the study? If not, then how can you possibly assume the opposite? They can always write the justification for choosing the hypothesis after the fact.

EDIT: This need not be done maliciously, that could very well be the actual reason why they decided to look at the birthday. What is concealed is how many other possibilities, equally well justified, were considered.