Strength of evidence

Section 9.1 of the Reviewers' Handbook discusses some things you may like to consider when discussing the strength of evidence in your review and you should read it now.

The phrase 'strength of evidence' applies to more than just the results of your review, although your review will obviously contribute greatly to the overall body of knowledge on its topic. It is how strong the overall case for the use or cessation of use for the intervention is. The strength of evidence relating to your review question is determined by factors both within your review and external to your review.

Internal factors

The pdf file to your left shows some examples of the things you need to consider when assessing the strength of the evidence you have collected in your review

External factors

• Biological plausibility. Your conclusion will be strengthened if the effect makes sense. There may well be laboratory based research to explain the effect demonstrated by your results, for example studies analyzing the biomechanics of hemiplegic gait may help explain evidence for the effect of a physiotherapy intervention following stroke. Similarly, the effect on intermediate outcomes, such as physiological markers, may help to explain and so strengthen the effect demonstrated by your review.
• Other evidence. There may be non-randomised studies, such as cohort studies or case series which are not included in your review but support your conclusions. This may strengthen the evidence. It is important to bear in mind however that these studies were excluded from your review for a reason (probably methodological) and so not too much weight should be given to their conclusions. In addition, you are unlikely to have systematically searched for these non-randomised studies and so you may not have all available information.
• Concordance with related reviews. If your review results are similar to other related reviews and the intervention appears effective (or not effective) in other comparable situations, this will strengthen your conclusions. For example, if an analgesic medication demonstrated similar pain relieving properties in a review of populations with acute pain and a review of populations with chronic pain, the strength of evidence for the intervention could be interpreted as stronger.

All of the above points need to be considered when helping the reader interpret the results of your review. Your conclusions about the effect of the intervention should reflect the strength of evidence as determined by these internal and external factors.