Risk and Numbers Needed to Treat

Revised and reviewed 26 August 2015


  • Risk is a nebulous term in evidence-based medicine, as it may refer to either relative or absolute risks
  • Overall, absolute risks are more meaningful and can be converted in ‘numbers need to treat’ (NNT), which are useful in the clinical setting


Absolute Risk

  • the actual event rate in the placebo or treatment group

Relative Risk

  • the ratio of the incidence of disease among exposed to the incidence among non-exposed
  • a measure of the strength of an association between groups
  • prospective studies (RCTs and cohort studies)
  • also called the incidence risk
Relative Risk

RR = risk of disease in the exposed (a/a+b) / risk of disease in the non-exposed (c/c+d)

  • RR of 3 -> there is three times the risk
  • RR of 0.5 -> the risk is halved
  • RR of 1 -> there is no association
  • if the RR is reported with a CI that includes 1 -> then the RR is not significant.

Relative Risk Reduction

  • expressed as a percentage reduction in events in treated vs untreated groups
  • = 1 – (incidence in exposed / incidence in unexposed)

Attributable Risk

  • a measure of association that provides information about the absolute effect of the exposure or excess risk of disease of those exposed compared to unexposed

Absolute Risk Reduction (ARR)

  • incidence in exposed – incidence in unexposed
  • a measure of treatment effect
  • reverse of attributable risk


  • NNT is the number of patients who need to be treated in order to avoid one adverse event, which is the reciprocal of the absolute risk reduction
  • NNT gives the RR some relevance in terms of the magnitude of clinical effect
  •  ie. if the incidence of an adverse event = 0.06% & relative risk reduction = 0.33 the absolute risk reduction -> 0.02% and thus numbers needed to treat = 1/0.0002 = 5000
  • however, if the incidence of an adverse event was 6% the absolute risk reduction -> 2% and thus the NNT = 1/0.02 = 50

CCC 700 6

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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