The questions addressed by a review may be broad or narrow in scope. For example, a review might address a broad question regarding whether antiplatelet agents in general are effective in preventing all thrombotic events in humans. Alternatively, a review might address whether a particular antiplatelet agent, such as aspirin, is effective in decreasing the risks of a particular thrombotic event, stroke, in elderly persons with a previous history of stroke.
Determining the scope of a review question is a decision dependent upon multiple factors including perspectives regarding a question’s relevance and potential impact; supporting theoretical, biologic and epidemiological information; the potential generalizability and validity of answers to the questions; and available resources.
There are advantages and disadvantages to both broad and narrow questions, some of which are summarized in Table 5.6.a. The validity of very broadly defined reviews may be criticized for ‘mixing apples and oranges’, particularly when good biologic or sociological evidence suggests that various formulations of an intervention behave very differently or that various definitions of the condition of interest are associated with markedly different effects of the intervention.
In practice, a Cochrane review may start (or have started) with a broad scope, and be divided up into narrower reviews as evidence accumulates and the original review becomes unwieldy. This may be done for practical and logistical reasons, for example to make updating easier as well as to make it easier for readers to keep up to date with the findings. Individual authors in consultation with their CRGs must decide if there are instances where splitting a broader focused review into a series of more narrowly focused reviews is appropriate and the methods that are implemented to achieve this (See Chapter 3, Section 3.4.4). If a major change is to be undertaken, such as splitting a broad review into a series of more narrowly focused reviews, a new protocol will need to be published for each of the component reviews which clearly document the eligibility criteria for each one.
The advent of Cochrane Overviews of reviews (Chapter 22), in which multiple Cochrane reviews are summarized, may affect scoping decisions for reviews. Overviews can summarize multiple Cochrane reviews of different interventions for the same condition, or multiple reviews of the same intervention for different types of participants. It may increasingly be considered desirable to plan a series of reviews with a relatively narrow scope, alongside an Overview to summarize their findings.