The practice of intention-to-treat

These two examples should give a clear indication of the dangers of compromising the randomisation at the analysis stage. Even small departures in terms of excluding patients from the analysis could have a major impact on the validity of the conclusions. 

The principle of intention-to-treat (ITT) tells us to compare the patients according to the treatments to which they were randomised. Randomisation gives us comparable groups, removing patients at the analysis stage destroys the randomisation and introduces bias. Randomisation also underpins the validity of the statistical comparisons. If we depart from the randomisation scheme then the statistical properties of our tests are compromised. 

The FDA guideline on anti-microbial drugs captures the issues well. 

FDA (1998) Developing Anti-microbial Drugs - General Considerations for Clinical Trials  

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The term full analysis set was introduced in order to separate the practice of intention-to-treat from the principle, but practitioners still frequently use the term intention-to-treat population when referring to this set. The term modified intention-to-treat population is also in common use within particular companies and also by regulators in some settings where exclusions from strict intention-to-treat are considered. 

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