Intention to treat analysis

  • "Once randomised, always analysed"
  • All enrolled patients have to be a part of the final analysis
  • This preserves the bias-protective effect of randomisation
  • Contrasts with per-protocol analysis and as-treated analysis:
    • Per-protocol analysis: only includes patients who adhered to the protocol; subject to selection bias. May be appropriate for analysis of adverse effects in drug trials.
    • As-treated analysis: groups patients according to the treatment they received, rather than the treatment they were allocated to receive. Subject to confounding.
    • Modified intention-to-treat analysis: allows the exclusion of patients which never started treatment for whatever reason. Very subjective, and open to selection bias.

Advantages

  • A more reliable estimate of treatment effectiveness
  • Prevents bias
  • Minimises Type 1 errors (false positives)
  • Supported by the CONSORT statement
  • When intention-to-treat analysis agrees with per-protocol analysis, it increases the validity of the study

Disadvantages

  • Treatment effect is diluted (ends up underestimated)
  • ITT is inaccurate unless there are negligible protocol violations
  • ITT alone is inappropriate for non-inferiority trials