# RLO: Determining the clinical importance of trial results

## Resources

Confidence Intervals (RLO) Defines the term 'confidence intervals' and demonstrates how they can be used to determine the significance and range of possible sizes of a treatment effect.
Number needed to treat and number needed to harm (RLO) Considers how to measure and interpret the magnitude of effect in clinical trial results using number needed to treat (NNT) and number needed to harm (NNH).
Positive and predictive value of diagnostic tests (RLO) Explains how diagnostic test results are a combination of true and false positive, or true and false negative.
Relative risk reduction and absolute risk reduction (RLO) Considers how to measure and interpret the magnitude of effect in clinical trial results using relative risk reduction (RRR) and absolute risk reduction (ARR).
Surrogate Outcomes (RLO) Considers the type of evidence which should be used when making decisions about patient care.

## Glossary

• Surrogate Outcome - should reflect the treatment effect on an important clinical endpoint, but does not directly measure the clinical endpoint itself.
• Control event rate (CER) = proportion of patients who experience an outcome in the control group
• Experimental event rate (EER) = proportion of patients who experience an outcome in the experimental group
• Relative risk reduction (RRR) = difference in event rates relative to (or proportional to) the control event rate, expressed as a percentage. RRR = (CER - EER) / CER
• Absolute risk reduction (ARR) = difference between the control event rate and the experimental event rate, expressed as a percentage. ARR = CER - EER
• Number needed to treat (NNT) = the number of patients that would need to be treated in order for ONE on them to have the beneficial outcome. It is calculated by calculating the reciprocal (or inverse) of the ARR. NNT = 1 / ARR
Note: if the ARR is expressed as a percentage (%) then: NNT = 1/ARR x 100 or 100/ARR
• Number needed to harm (NNH) = the number of patients that would need to be treated in order for ONE on them to experience the adverse outcome. It is calculated by calculating the reciprocal (or inverse) of the absolute risk increase (ARI). NNH = 1/ARI
Note: if the ARI is expressed as a percentage (%) then NNH = 1/ARI x 100 or 100/ARI
• Confidence Interval – tells you the range of values within which the true value could feasibly lie, given the size of the difference observed.

RLO Transcript (Rich Text Format 61kb)

© 2007 School of Nursing and Academic Division of Midwifery, University of Nottingham

Developer: Neil Duffin

Content authors: Elaine Bentley, Julia Lacey, Briony Leighton

RLO released: 14th March, 2007

Page last updated: 29 March, 2021