Systematic review (SR) methodology is a structured, rigorous, and transparent form of literature evaluation that was first developed for evidence synthesis in clinical research. Its intention was to give policy makers and practitioners access to the current state of evidence and to help answer various clinical health questions. The SR methodology gained popularity over time in other science domains including social care and environmental management.
In the last two decades, it has been accepted as the highest standard method of knowledge assessment across various research fields. It is now adopted by healthcare organisations as well as governmental bodies and research institutes across an increasing number of scientific disciplines to further research and to help translate relevant knowledge into various policies, guidelines, and practices.
The SR methodology is powerful because it eliminates bias more effectively than narrative reviews. Like a narrative review, the SR methodology follows a set of core steps to evaluate and summarise the outcomes of individual research studies carried out by others.
The difference is that the SR methodology demands extra layers of processing and documentation to increase rigor, transparency, and reproducibility. The extra processing includes the quality assessment, also called as "risk of bias " assessment, for each study that is included in the review as well as for the whole body of evidence. Additionally, depending on the amount and quality of the available data, statistical analysis may be used to enhance the power of the overall data in a single analysis which is commonly referred to as meta-analysis.
The SR methodology also requires a high level of consistency and transparency which is made possible through following a pre-defined and documented procedure, known as the SR protocol, that is driven from the review question. The protocol defines the key elements of the review question and outlines the review methods in advance, to protect the outcome from bias.
The SR methodology is applicable and effective in answering well defined and focused research questions, containing all necessary elements that enable the verification of their outcomes. Broad research questions need to be refined and framed into measurable units before they can be answered by the method. (See supplementary materials for details)
The extra layers of processing and documentation often demand high degrees of teamworking between reviewers with knowledge of the topic and methodology. Sufficient budget and time are also needed. The need for these resources can constrain a researcher’s ability to properly apply the methodology, and thus the lack of proper resources can limit the appropriateness of the method.
More importantly, the effectiveness of the SR methodology relies on the quality and quantity of existing research. In other words, the outcomes of systematic reviews are only useful when enough high-quality primary research are available for review. Apart from lack of data, various weaknesses in conduct and/or reporting of primary studies might make the outcomes of systematic reviews inconclusive.
Ultimately, like any other research studies systematic reviews are prone to bias of conduct and reporting. The quality of systematic reviews relies heavily on the knowledge and expertise of the reviewers and their adherence to the principles of rigor, comprehensiveness, transparency and reproducibility.