What is an intention to treat analysis?
Intention to treat analyses are prevalent in clinical trials. Hollis and Campbell (1999) present an overview of their use. The object is to use data from all subjects originally allocated to the groups at the outset of the trial (ie who were intended to be treated in the trial) irrespective of whether they dropped out or staisfied entry criteria.
Failure to include all patients can underestimate the effectiveness of a treatment. For example suppose some patients are given a memory aid which includes a reminder to 'take your medication'. The patients are not reminded of this due to a failure to use the memory aid correctly and then withdraw from the study for health reasons. This represents informative dropout in that the reason for the dropout is related to the effectiveness of thge memory aid being assessed. If these patients are excluded the usefulness of the memoru aid may be overestimated.
Reference
[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC28218/?tool=pmcentrez Hollis S and Campbell F (1999) What is meant by intention to treat analysis? Survey of published randomised controlled trials. BMJ 319(7211) 670-674.]