NCCMT - URE - Effectiveness of Interventions - Understanding the Number Needed to Treat

Video created: May 8, 2013

When looking at research evidence to support your public health decisions, you may come across multiple interventions that are shown to be effective. The number needed to treat (NNT) tells you how many people need to receive an intervention in order for you to see one person with the desired outcome. This important information can help you decide which intervention will deliver the greatest impact.

This video explains how to calculate the NNT and how to interpret it. Generally, the smaller the NNT, the greater impact the intervention will have on the population. However, it is also important to consider specific population risks, which may affect the implementation and outcomes of an intervention.

Variations of the NNT are also covered in this video, including the number needed to harm (NNH) and the number needed to immunize (NNI). The NNH tells you how many people would need to receive an intervention before an unintended negative outcome could be expected. In this case, you would hope for a large number. The number needed to immunize tells you how many people need to be immunized in order to avoid one negative outcome.

Although systematic reviews do not usually include the NNT, most published studies provide enough data on the outcome measures so that you can calculate the NNT yourself. Knowing the NNT of an intervention can help you make informed decisions about where to invest scarce public health programming resources.

The National Collaborating Centre for Methods and Tools is funded by the Public Health Agency of Canada and affiliated with McMaster University. The views expressed herein do not necessarily represent the views of the Public Health Agency of Canada.

NCCMT is one of six National Collaborating Centres (NCCs) for Public Health. The Centres promote and improve the use of scientific research and other knowledge to strengthen public health practices and policies in Canada.