Having identified and selected the studies for your systematic review, the next step is to extract the relevant data from the full-text using a standardised extraction form. The purpose of data extraction is to accurately summarise the studies in a format that will facilitate synthesis - for example, identifying the population, methodology, outcomes, etc.
Cornell University Library's guide on 'Data extraction' provide some useful tips on this stage of a systematic review.
"'Narrative synthesis’ refers to an approach to the systematic review and synthesis of findings from multiple studies that relies primarily on the use of words and text to summarise and explain the findings of the synthesis. Whilst narrative synthesis can involve the manipulation of statistical data, the defining characteristic is that it adopts a textual approach to the process of synthesis to ‘tell the story’ of the findings from the included studies".
Popay et al. (2006). Guidance on the Conduct of Narrative Synthesis in Systematic Reviews document
Cochrane Training have produced a five videos series "Narrative synthesis" of quantitative effect data in Cochrane reviews: current issues and ways forward. These are aimed at review authors and editors and are about improving transparent reporting of synthesis without meta-analysis of effect sizes.
See also the SWiM (Synthesis Without Meta-Analysis) website: https://swim.sphsu.gla.ac.uk/
"Meta-analysis is a statistical technique for combining data from multiple studies on a particular topic".
The What is meta-analysis? PDF from the 'What is' series provides a useful overview of meta-analysis. This includes sections on: What is meta-analysis?, benefits of meta-analyses, requirements for meta-analysis, conducting meta-analyses, presenting the findings, and limitations.
Crombie, I. K. & Davies, H. T. (2009). What is meta-analysis (2nd ed.). Haywood Medical Publications.
PRISMA is an evidence-based minimum set of items for reporting in systematic reviews and meta-analyses. PRISMA focuses on the reporting of reviews evaluating randomised trials, but can also be used as a basis for reporting systematic reviews of other types of research, particularly evaluations of interventions.
The PRISMA checklist - this 27 item checklist details the qualities a high-quality paper should contain.
The PRISMA Flow Diagram - designed to be used for researchers to depict the flow of information through the different phases of a systematic review. It maps out the number of records identified, included and excluded, and the reasons for exclusions. See example below of the PRISMA flow diagram for new systematic review which included searches of databases and registers only.
The PRISMA Extension for Searching - published in 2021, the checklist includes 16 reporting items, each of which is detailed with exemplar reporting and Rationale.