How Clinical Evidence works
The Clinical Evidence website (www.clinicalevidence.bmj.com) summarises the current state of knowledge and uncertainty about interventions used for prevention and treatment of important clinical conditions. To achieve this, we systematically search and appraise the world literature to provide rigorous systematic reviews of evidence on the benefits and harms of clinical interventions.
Making summaries involves excluding some detail, and users of Clinical Evidence need to be aware of the limitations of the evidence presented. It is not possible to make global statements that are both useful and apply to every patient or clinical context that occurs in practice. For example, when stating that we found evidence that a drug is beneficial, we mean that there is evidence that the drug has been shown to deliver more benefits than harms when assessed in at least one group of people, using at least one outcome at a particular point in time. It does not mean that the drug will be effective in all people given that treatment or that other outcomes will be improved, or even that the same outcome will be improved at a different time after the treatment.
Our categorisation of interventions
Each systematic review contains a page that lists key clinical questions and interventions and describes whether they have been found to be effective or not.
We have developed these categories of effectiveness from one of the Cochrane Collaboration’s first and most popular products, A guide to effective care in pregnancy and childbirth. The categories are explained in the table below.
| Intervention | Icon | Description |
|---|---|---|
| Beneficial |  |
For which effectiveness has been demonstrated by clear evidence from systematic reviews, RCTs, or the best alternative source of information, and for which expectation of harms is small compared with the benefits. |
| Likely to be beneficial |  |
For which effectiveness is less well established than for those listed under “beneficial”. |
| Trade off between benefits and harms |  |
For which clinicians and patients should weigh up the beneficial and harmful effects according to individual circumstances and priorities. |
| Unknown effectiveness |  |
For which there are currently insufficient data or data of inadequate quality. |
| Unlikely to be beneficial |  |
For which lack of effectiveness is less well established than for those listed under “likely to be ineffective or harmful”. |
| Likely to be ineffective or harmful |  |
For which ineffectiveness or associated harm has been demonstrated by clear evidence. |
Fitting interventions into these categories is not always straightforward. For one thing, the categories represent a mix of several hierarchies: the size of benefit (or harm), the strength of evidence (RCT or observational data), and the degree of certainty around the finding (represented by the confidence interval). Another challenge is that much of the evidence most relevant to clinical decisions relates to comparisons between different interventions rather than to comparison with placebo or no intervention. Where necessary, we have indicated the comparisons. A third consideration is that interventions may have been tested, or found to be effective, in only one group of people, such as those at high risk of an outcome. Again, we have indicated this where possible. But perhaps most difficult of all is trying to maintain consistency across different systematic reviews. We continue to work on refining the criteria for categorising interventions. Interventions that cannot be tested in an RCT for ethical or practical reasons are sometimes included in the categorisation table and are identified with an asterisk.