Intention to Treat
46Weekend Editor
24Viliam
18jimrandomh
16simon
2Hastings
1Pat Myron
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The way I used to explain "as treated" vs "intent to treat":
Under AT, you're asking a question about biology: does this treatment make a difference when it's applied rigorously to the tests and absolutely never to the controls?
Under ITT, you're asking a question about the practice of medicine, under actual combat conditions: if we tell clinicians and patients to do something, and they follow instructions as imperfectly as ever, is this treatment still a good idea?
That is: ITT bakes in mistakes, noncompliance, weird patients, etc. on top of the basic scientific effect.
8This can be quite frustrating if you want to figure out "what happens if I do X", and all the answers provided by science turn out to be about "what happens if people kinda want to do X, but then most of them don't".
I mean, it is good and potentially important to know that most people who kinda want to do X will fail to actually do it... but it doesn't answer the original question.
1When I read studies, the intention-to-treat aspect is usually mentioned, and compliance statistics are usually given, but it's usually communicated in a way that lays traps for people who aren't reading carefully. Ie, if someone is trying to predict whether the treatment will work for their own three year old, and accurately predicts similar compliance issues, they're likely to arrive at an efficacy estimate which double-discounts due to noncompliance. And similarly when studies have surprisingly-low compliance, people who expect themselves to comply fully will tend to get an unduly pessimistic estimate of what will happen.
Also, when doing a study, please write down afterwards whether you used intention to treat or not.
Example: I encountered a study that says post meal glucose levels depend on order in which different parts of the meal were consumed. But the study doesn't say whether every participant consumed the entire meal, and if not, how that was handled when processing the data. Without knowing if everyone consumed everything, I don't know if the differences in blood glucose were caused by the change in order, or by some participants not consuming some of the more glucose-spiking meal components.
In that case, intention to treat (if used) makes the result of the study less interesting since it provides another effect that might "explain away" the headline effect.
Furthermore, even if what you care about is the effect of treatment with perfect compliance, the effect of intention to treat is still probably the first statistic you should look at. The most important point of intention to treat is that it is likely to measure the sign of the effect correctly, whereas a naive "as treated" measurement often won't due to selection bias. (And, cynically, non-naive "as treated" measurement will tell whatever story the authors want it to tell)