Systematic summaries

Consider whether summaries were systematic.

A systematic review is a summary of studies that addressed a specific question. It uses careful and clear steps to find relevant studies, assess the trustworthiness of those studies, and put together the results of those studies.


The results of a single study of a health action can be misleading. It is important to consider all the relevant fair comparisons when deciding what to believe about the effects of a health action. Systematic reviews can help to ensure that this is done. Unfortunately, many reviews of the effects of health actions are not systematic. Even reviews that claim to be systematic may not be. Reviews that are not systematic may not include all the relevant fair comparisons, they may assess the trustworthiness of studies unfairly, and the way in which the results of included studies are put together may be misleading.

The starting point for a systematic review is a clear question. This includes clearly specifying the people who might be affected by the health action (e.g., people with a specific disease or at risk of a disease), the health action, what the health action is compared to (e.g., not taking the health action or taking another health action), and the outcomes (good or bad things that might happen because of the health action) that are important to the people who are affected. Specifying these things is the basis for deciding which studies should be included, how to search for those studies, what information to collect from the included studies, and how to put together the results of the included studies.

Most of the information about health actions on the internet is not based on systematic reviews and there is a lot of misinformation. It may be difficult for many people (including health professionals) to find systematic reviews on the effects of a health action. This can be helped by user-friendly sources of information about the effects of health actions that is based on reliable, up-to-date systematic reviews. Whenever possible, people should rely on sources of information about health actions that are based on systematic reviews (e.g., When this is not possible, people should ask whether a claim about the effects of a health action is based on a systematic review.


In the late 1980s, the results of a very large randomized trial showed that using blood thinning drugs in patients with an acute heart attack reduced the number of deaths. However, the effectiveness of this treatment could have been recognized 10 years earlier if a systematic review had been done of the evidence available at that time, and thousands of deaths could have been avoided. Instead, recommendations were based on reviews of the evidence that were not systematic.

Remember: Whenever possible, use up-to-date systematic reviews of fair comparisons to help you decide what to do or believe, rather than single studies or reviews that are not systematic.

Educational resources for this concept
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