"ITT analysis includes every subject who is randomized according to randomized treatment assignment. It ignores noncompliance, protocol deviations, withdrawal, and anything that happens after randomization. ITT analysis is usually described as “once randomized, always analyzed”.
"ITT analysis avoids overoptimistic estimates of the efficacy of an intervention resulting from the removal of non-compliers by accepting that noncompliance and protocol deviations are likely to occur in actual clinical practice" - Gupta, 2011
In Mastering Metrics, Angrist and Pischke describe intent-to-treat analysis:
"In randomized trials with imperfect compliance, when treatment assignment differs from treatment delivered, effects of random assignment...are called intention-to-treat (ITT) effects. An ITT analysis captures the causal effect of being assigned to treatment."
While treatment assignment is random, non-compliance is not! Therefore if instead of using intent to treat comparisons we compared those actually treated to those untreated we would get biased results, because this is essentially making uncontrolled comparisons between treated and untreated subjects.
Angrist and Pishke describe how instrumental variables can be used in this context:
“The instrumental variables (IV) method harnesses partial or
incomplete random assignment, whether naturally occurring or generated
"Instrumental variable methods allow us to capture the causal effect of treatment on the treated in spite of the nonrandom compliance decisions made by participants in experiments....Use of randomly assigned intent to treat as an instrumental variable for treatment delivered eliminates this source of selection bias."
In Intent-to-Treat vs. Non-Intent-to-Treat Analyses under Treatment Non-Adherence in Mental Health Randomized Trials there is a nice discussion of ITT and IV methods with applications related to clinical research. Below is a nice treatment of IV in this context:
“Instrumental variables are assumed to emulate randomization variables, unrelated to unmeasured confounders influencing the outcome. In the case of randomized trials, the same randomized treatment assignment variable used in defining treatment groups in the ITT analysis is instead used as the instrumental variable in IV analyses. In particular, the instrumental variable is used to obtain for each patient a predicted probability of receiving the experimental treatment. Under the assumptions of the IV approach, these predicted probabilities of receipt of treatment are unrelated to unmeasured confounders in contrast to the vulnerability of the actually observed receipt of treatment to hidden bias. Therefore, these predicted treatment probabilities replace the observed receipt of treatment or treatment adherence in the AT model to yield an estimate of the as-received treatment effect protected against hidden bias when all of the IV assumptions hold.”
A great example of IV and ITT applied to health care can be found in Finkelstein et. al. (2013 & 2014) - See the Oregon Medicaid Experiment, Applied Econometics, and Causal Inference.
Over at the Incidental Economist, there was a nice discussion of ITT in the context of medical research that does a good job of explaining the rationale as well as when departures from ITT make more sense (such as safety and non-inferiority trials).
Instrumental Explanations of Instrumental Variables
A Toy IV Application
Other IV Related Posts
The Path from Cause to Effect
Joshua D. Angrist & Jörn-Steffen Pischke
Gupta, S. K. (2011). Intention-to-treat concept: A review. Perspectives in Clinical Research, 2(3), 109–112. http://doi.org/10.4103/2229-3485.83221
Ten Have, T. R., Normand, S.-L. T., Marcus, S. M., Brown, C. H., Lavori, P., & Duan, N. (2008). Intent-to-Treat vs. Non-Intent-to-Treat Analyses under Treatment Non-Adherence in Mental Health Randomized Trials. Psychiatric Annals, 38(12), 772–783. http://doi.org/10.3928/00485713-20081201-10
"The Oregon Experiment--Effects of Medicaid on
Clinical Outcomes," by Katherine Baicker, et al. New England Journal of
Medicine, 2013; 368:1713-1722.
Medicaid Increases Emergency-Department Use:
Evidence from Oregon's Health Insurance Experiment. Sarah L.
Taubman,Heidi L. Allen, Bill J. Wright, Katherine Baicker, and Amy N.
Finkelstein. Science 1246183Published online 2 January 2014
Detry MA, Lewis RJ. The Intention-to-Treat PrincipleHow to Assess the True Effect of Choosing a Medical Treatment. JAMA. 2014;312(1):85-86. doi:10.1001/jama.2014.7523