1. Introduction

A protocol for carrying out a review is equivalent to, and as important as, a protocol for a primary research study. A review is less likely to be biased if the questions are well developed beforehand, and the methods that will be used to answer them are decided on before gathering the necessary data and drawing inferences. In the absence of a protocol, it is possible that study selection and analysis will be unduly driven by (a presumption of) the findings.

A search strategy consists of several aspects. The research question (in a structured format, see Building a search question) should be used as a guide to direct the search strategy. For electronic searches, it is important to list the databases in which studies will be sought. Other sources can be consulted in order to identify all relevant studies. These include reference lists from relevant primary and review articles, journals, grey literature and conference proceedings, research registers, researchers and manufacturers, and the internet.

In practice, it is uncommon for a single search to cover all the questions being addressed within a review. Different questions may be best answered by different databases, or may rely on different study types. Authors are encouraged to take an iterative approach to the search, carrying out a search for high-level evidence first. After evaluating the results of this first search, the questions may need to be redefined and subsequent searches may need to be focused on more appropriate sources and study types.

In some cases, directly relevant good-quality evidence syntheses (secondary sources), such as good-quality systematic reviews or Health Technology Assessments (HTA), will be available on some of the issues that fall within the remit of the review. In these circumstances reference will be made to the existing evidence rather than repeating work that already has been done. All HTA reports or systematic reviews that are identified must be evaluated on their quality and must be shown to have followed an acceptable methodology before they can be considered for use in this way.

In other cases existing evidence may not be directly relevant or may be found to have methodological weaknesses. In these cases, existing evidence cannot be used in the review. Nevertheless, excluded systematic reviews or HTA reports still can be a useful source of references that might be used later on in the review.

In conclusion, literature searches for the KCE should follow an iterative approach, searching for evidence syntheses first and subsequently complementing this search by searching for original studies. Various resources are listed in the following paragraph.