Does Intention-to-Treat reduce bias in a non-randomised trial? I am involved in a trial comparing the withdrawal patients staying in an in-patient unit experience after discontinuing either of two drugs. The drug they were taking before they commenced abstinence will not be not randomised.
A reviewer on the ethics committee who are assessing the study enquired about whether we were following intention-to-treat principles.
I never even considered using intention to treat since:
(i) participants cannot switch drugs during treatment since they are only being measured after they have stopped
(ii) participants were not randomised, the decision to use either drug was based on the preferences of the prescribing doctor and the participant.
But just say participants decide the withdrawal they experience is too intense and want to resume use of either the drug they were originally on or the other drug. If they agree to continue providing withdrawal symptom data should their withdrawal symptom data still be analysed as if they had never resumed use of the drug?
From what I understand from here and here intention to treat preserves the bias-removal effect inherent in randomisation.
But does intention to treat offer the same advantages when there is no randomisation?
As you can tell I am a bit confused and would welcome some clarity.
 A: I have only seen the term "intent to treat" in randomized trials, where you are analyzing the effect of treatment assignment. Not sure if it is the right term to use in this case.
As for the ethical issues, perhaps they are worried these patients do not voluntarily discontinue?
Comparing the withdrawal patients from two different drugs in non-randomized setting sounds a very complicated analysis to me. You need to make sure the two groups are comparable in the first place after controlling for the confounding factors. Otherwise the results are difficult to interpret. Not sure if it is feasible in this case. Even if the two groups of patients are from a randomized trial for two drugs, the withdrawal patients are not comparable.
Should their withdrawal symptom data still be analysed as if they had never resumed use of the drug? Yes, I think so. If you are interested in the withdrawal symptoms, the data after resumed use of the drug may not be useful. Perhaps you can also do a time to failure analysis and consider the resumed use as a failure. That being said, I am not sure how to design this analysis properly in the first place.
A: I think intention-to-treat should be considered in non-randomly assigned groups. Just thinking about a study of a programme to change behaviour (smoking). Group A nominated themselves for the programme, but 40 out of 50 of them did not complete it, it was sooo boring. It would be unfair to compare the 10 who did complete with the comparison group. The Intention-to-Treat principle is important because drop-outs skew your data, and if your intervention/drug is to be assessed comprehensively surely you need findings that account for the likelihood that people don't finish the intervention/drug prescription.
