Systematic reviews are a type of meta-research (research on research) in which primary research studies (e.g., randomised controlled trials) are included to answer a specific research question by synthesising the available evidence. They differ from narrative reviews as they are conducted in a standardised way, and the results and procedures could be replicated. Early systematic reviews were employed to study the efficacy of clinical treatments, and then they started to be used to address different questions about experiences, diagnostics, and prognostics. These days systematic reviews synthesise qualitative evidence too and some even include both quantitative and qualitative methods.
Several aspects are considered before beginning a systematic review, for instance, whether the research question has been answered already, and if so whether the systematic review needs updating, or could be conducted in a different way. If the research question is about addressing gaps in the literature or map out the evidence in different areas, then a scoping review is the best type. If an answer needs to be given quickly to an emerging topic, it may be worth conducting a rapid review. If the body of research already has systematic reviews, then, a systematic overview which will include several systematic reviews is conducted. Other aspects to consider at the planning stage are whether there is a need for a statistician to analyse the results, especially if a meta-analysis is conducted to obtain an overall effect size, and whether a methodologist is needed to check for bias within studies.
When searching and selecting the literature or creating the protocol, the PICOS or SPIDER summaries are used to help thinking about the suitability of the studies, and to identify the search terms. The PICOS summary is used for quantitative research, and it stands for: Participants, Intervention, Comparison, Outcome, (type of) Study (e.g., observational, cohort, cross-sectional, case study). The SPIDER summary is employed for qualitative research, and it stands for: Sample, Phenomenon of Interest, Design (e.g., interview) Evaluation (e.g., experiences), Research Type (e.g., qualitative, or mixed method).
To avoid bringing bias to the systematic review and make the process objective and transparent it is important to consider whether the search is limited to a specific year range, and whether is limited to English or more languages. Regarding the databases searched, they will have to be reported, and researchers usually search a minimum of two. Another way to bring more rigour to the process is to use a checklist of evidence-based items like the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).
At the screening stage two reviewers decide which studies are included and which are excluded based on criteria agreed beforehand. Data extraction is the next stage, so the characteristics of the included studies are reviewed. Afterwards, the data is analysed and so the included studies are compared to see how homogeneous they are in terms of their designs, interventions, outcomes, and type of participants. On the last stage the quality of the overall evidence is assessed.
Systematic reviews add value to research. During the research planning they highlight whether more research is needed to confirm previous findings, and they point out any gaps in the research literature. In this way, systematic reviews help to avoid conducting unnecessary research, which in turn lead to a more efficient use of resources (e.g., by not wasting participants’ time, or exposing them to unnecessary treatments, and using research materials and funding in a more efficient manner). Systematic reviews add value to practice by helping to make informed decisions that are evidence-based, as they consider the evidence of a body of research set in a wider context, as opposed to considering results from just recent studies or few studies.