12.7.4  Common errors in reaching conclusions

A common mistake when there is inconclusive evidence is to confuse ‘no evidence of an effect’ with ‘evidence of no effect’. When there is inconclusive evidence, it is wrong to claim that it shows that an intervention has ‘no effect’ or is ‘no different’ from the control intervention. It is safer to report the data, with a confidence interval, as being compatible with either a reduction or an increase in the outcome. When there is a ‘positive’ but statistically non-significant trend authors commonly describe this as ‘promising’, whereas a ‘negative’ effect of the same magnitude is not commonly described as a ‘warning sign’; such language may be harmful.


Another mistake is to frame the conclusion in wishful terms. For example, authors might write “the included studies were too small to detect a reduction in mortality” when the included studies showed a reduction or even increase in mortality that failed to reach conventional levels of statistical significance. One way of avoiding errors such as these is to consider the results blinded; i.e. consider how the results would be presented and framed in the conclusions had the direction of the results been reversed. If the confidence interval for the estimate of the difference in the effects of the interventions overlaps the null value, the analysis is compatible with both a true beneficial effect and a true harmful effect. If one of the possibilities is mentioned in the conclusion, the other possibility should be mentioned as well.


Another common mistake is to reach conclusions that go beyond the evidence. Often this is done implicitly, without referring to the additional information or judgements that are used in reaching conclusions about the implications of a review for practice. Even when additional information and explicit judgements support conclusions about the implications of a review for practice, review authors rarely conduct systematic reviews of the additional information.  Furthermore, implications for practice are often dependent on specific circumstances and values that must be taken into consideration. As we have noted, authors should always be cautious when drawing conclusions about implications for practice and they should not make recommendations.