Intention-to-treat concept
One practical problem that investigators usually come across in RCT (randomized controlled trials) is that subjects do not always follow instructions. (medical drug experiments). Hence, RCT often suffers from two major complications, i.e., noncompliance and missing outcomes.
ITT analysis includes every subject who is randomized according to randomized treatment assignment, regardless of the treatment they actually received, and regardless of subsequent withdrawal from treatment or deviation from the protocol.
ITT analysis avoids overoptimistic estimates of the efficacy of an intervention.
IIT analysis reduces type I error.
Many arguments against ITT analysis appear valid. To begin with, if a subject who actually did not receive any treatment is included as a subject who received treatment, then it indicates very little about the efficacy of the treatment. In ITT analysis, estimate of treatment effect is generally conservative because of dilution due to noncompliance.
ITT analysis has been criticized for being too cautious and thus being more susceptible to type II error.
Use IIT as complementary method.
"When the ITT and per-protocol (PP) analyses come to essentially the same conclusions, confidence in the study results is increased."
PP analysis is defined as a subset of the ITT population who completed the study without any major protocol violations.