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.
Care should be taken to minimise missing responses. If missing responses do occur the most common ways of handling these are using complete cases and carrying forward the last recorded response (last observation carried forward). The latter methods has the advantage of including all the patients and can be seen as a conservative assessment of change in that improvement may be expected to get more pronounced over time. For example suppose we are interested in improvement over time in recall using a memory aid. It might be expected that with practice people might recall more events using the memory aid so that using earlier responses denoting the number of items remembered may underestimate the number that would ahve been remembered later with more practice using the memory aid in those who had missing data.
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.]