While it is important to be able to read and interpret individual papers, the results of a single study are never going to provide the complete answer to a question. To move towards this we need to review the literature more widely. There can be a number of reasons for doing this, some of which require a more comprehensive approach than others. If the aim is simply to increase our personal understanding of a new area then a few papers might provide adequate background material. Traditional narrative reviews, which give less emphasis to complete coverage of the literature and tend to be more qualitative, have value for exploring areas of uncertainty or novelty, but it is harder to scrutinise them for flaws. In contrast, a major policy decision might require a systematic review of all the relevant literature. We will focus on the systematic approach here, but this can of course be tailored according to need.

What is a systematic review?

Like a primary research paper, an epidemiological review should aim to produce a helpful synthesis of primary data – looking for patterns but not hiding differences – and it should normally offer a formal causal interpretation. Although its primary data units are whole studies rather than individuals, the review process should have the same rigour as its component studies. A systematic review should be a response to a clearly formulated question and involve the identification of all relevant primary research studies that address that question.