As with any research, the first and most important decision in preparing a systematic review is to determine its focus. This is best done by clearly framing the questions the review seeks to answer. Well-formulated questions will guide many aspects of the review process, including determining eligibility criteria, searching for studies, collecting data from included studies, and presenting findings (Jackson 1980, Cooper 1984, Hedges 1994) . In Cochrane reviews, questions are stated broadly as review ‘Objectives’, and specified in detail as ‘Criteria for considering studies for this review’. As well as focussing review conduct, the contents of these sections are used by readers in their initial assessments of whether the review is likely to be directly relevant to the issues they face.
A statement of the review’s objectives should begin with a precise statement of the primary objective, ideally in a single sentence. Where possible the style should be of the form ‘To assess the effects of [intervention or comparison] for [health problem] in [types of people, disease or problem and setting if specified]’. This might be followed by one or more secondary objectives, for example relating to different participant groups, different comparisons of interventions or different outcome measures.
The detailed specification of the review question requires consideration of several key components (Richardson 1995, Counsell 1997). The ‘clinical question’ should specify the types of population (participants), types of interventions (and comparisons), and the types of outcomes that are of interest. The acronym PICO (Participants, Interventions, Comparisons and Outcomes) helps to serve as a reminder of these. Equal emphasis in addressing each PICO component is not necessary. For example, a review might concentrate on competing interventions for a particular stage of breast cancer, with stage and severity of the disease being defined very precisely; or alternately focus on a particular drug for any stage of breast cancer, with the treatment formulation being defined very precisely.