Or, people who were assigned a professional flosser every single school day were also more likely to brush their teeth better and were reminded constantly about their teeth and even possibly taught stuff about teeth and so maybe even ate less sugary things. I don't have access to the paper: did they at least have the professional flosser supervise the self-flossing to control for this? Otherwise this just seems like a much more likely reason for these results.
I can't find a PDF of the original study, but in the review, they mention that one quadrant of the mouth was professionally flossed. This presumably lets them do within-subject comparisons: are there more/less cavities (or whatever) within the flossed portion?
This should remove confounds like 'ate more sugary things'. It's possible, of course, that kids brushed more on the flossed side (or the unflossed side), but you could presumably quantify that too.
The reference is a systematic review, not a single study; only one study in the review was run as you indicate.
Beyond that, I am not sure what you are getting at. The point of the post I posted and the original article here is that self-flossing as an augmentation to brushing does effectively nothing. What you stated corroborates that. Am I missing something?
My point was that from the information in the article that isn't actually clear; it is like when people say that condoms are X% effective as a contraception technique, and so people assume that means that condoms have an X% failure rate, but what they really mean is that "couples who report using condoms as their primary means of contraception have an X% success rate at avoiding pregnancy". While it is interesting from a clinical perspective as to whether "sending a child home and telling them to floss by themselves" is a working strategy or not, it would be taking into account "children don't really floss" and doesn't tell us if an adult who actually knows they are flossing (such as an adult who actually knows they are using the condom every time) has a particular success rate.
