I have to point out the obvious confounder here. The kids who get their tonsils and adenoids out are the ones with recurrent respiratory infections to begin with. Really the only way to control for this is to randomize kids to surgery or not and then follow for 20 years or more. This in contrast is an observational study, albeit with a very large number of subjects. They said they tried to control for confounding factors, but it's hard to see how effective that can be after the fact.
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