Last updated: August 11th 2020
Some primary studies provide evidence of higher reliability and relevance than others in respect to the review question. Assessing the comparative validity of the included studies (often referred to as critical appraisal) is of key importance to the resulting value of the Systematic Review (see examples in Box 8.1 and Table 8.1). It can form a basis for the differential weighting of studies in later synthesis or partitioning of studies into subgroups for separate analyses.
Study validity assessment requires a number of decisions about the absolute and relative importance of different sources of bias and data validity elements common to environmental data, particularly the appropriateness of temporal and spatial scales. It is therefore vital that the assessment process be standardised and as transparent and repeatable as possible. This challenge has been extensively covered in the Planning Section (Section 3). Some extra points are made below that may help in the conduct as well as the planning stages.
In an ideal world, each data set included in a SR should be of high internal validity, thus ensuring that the potential for error and bias is minimised and that any differences in the outcome measure between experimental groups can be attributed to the exposure or intervention of interest. To determine the level of confidence that may be placed in selected data sets, the methodology employed to generate each one must be critically appraised, using a transparent and consistent framework, to assess the extent to which it is likely to prevent systematic errors or bias (Moher et al. 1995). However, the nature of the critical appraisal and the hierarchy employed is dependent on the nature of the question and the ‘theory of change’. The Review Team should be able to justify their approach and not blindly follow an established methodology.
In the health sciences, a hierarchy of research methodology is recognised that scores the value of the data in terms of the scientific rigour; the extent to which the methodology seeks to minimise error and bias (Stevens & Milne 1997). The hierarchy of methodological design can be viewed as generic and has been translated from medicine to environmental sciences (Pullin & Knight 2003), but these generic hierarchies are crude tools and usually just a starting point and can rarely be used without modification to ensure relevance to individual review questions. Where a number of well-designed, high-validity studies are available, others with inferior methodology may be demoted from subsequent quantitative analysis to narrative tabulation, or rejected from the SR entirely. However, there are dangers in the rigid application of hierarchies as the importance of various methodological dimensions within studies will vary, depending on the study system to which an intervention is being applied. For example, a rigorous methodology, such as a randomised controlled trial (RCT), applied over inadequately short time and small spatial scales could be viewed as superior to a time series experiment providing data over longer time and larger spatial scales that were more appropriate to the question. The former has high internal validity but low external validity or generalisability in comparison to the latter. This problem carries with it the threat of misinterpretation of evidence. Potential pitfalls of this kind need to be considered at this stage and explored in covariate analyses (e.g. experimental duration or study area: see Downing et al. 1999 and Côté et al. 2001, respectively) or by judicious use of sensitivity analysis at the synthesis stage (see below).
As a consequence, authors may use existing checklists of critical appraisal tools as a basis for their specific exercise, but they should either explain why they use them as such (no modification, because not considered to be needed, and why) or adapt them to their own case-study review, in which case the decisions made must be stated and justified (see Gough et al. 2012).
We suggest that review-specific a priori assessment criteria for appraising the internal validity are included in the Protocol and two or more assessors should appraise each study. The subjective decisions may be a focus of criticism; thus, we advocate consultation with subject experts and relevant stakeholders when planning your approach. Pragmatic grouping of studies into high, medium and low validity based on simple but discriminatory checklists of “desirable” study features may be necessary if sample sizes are small and do not allow investigation of all the study features individually (for example, Felton et al. 2010, and Isasi-Catalá 2010).
The scope of CEE Systematic Reviews is broad and often interdisciplinary and therefore we seek to be inclusive of different forms of evidence provided their strengths and weaknesses are properly appraised and comparative study weightings are appropriate. However, alongside this inclusivity we expect high levels of transparency providing details of the critical appraisal criteria, how they were applied and the judgements on validity of each study. Normally the full dataset will be provided as an additional supplementary file (see Section 10).
External validity is often considered in terms of the relevance of the study; how transferable is it to the context of the question? As noted above, some studies can be of high internal validity (low risk of bias) but may be misleading on account of low external validity (low relevance). A simple example is a high validity study that has been conducted outside the geographical region or in a slightly different ecosystem than the one of interest.
Appraisal of study relevance can be a more subjective exercise than appraisal of study reliability. Estimating the external validity of a study may require the construction of review-specific criteria formed by fit to the question elements or similar subjective measures (see Gough et al. 2012 for examples).
For transparency of reporting, tables of study validity assessment should be included as an appendix or supplementary material. The data validity assessment can be incorporated in narrative synthesis tables if appropriate. Sufficient text should be provided to enable the reader to navigate the tables and understanding the coding and appraisal methods used.
Table 8.1. Elements of a data validity assessment of studies included in a SR examining impacts of land management on carbon cycling and greenhouse gas fluxes in boreal and temporal lowland peats.
At the end of this stage (if not before) it should become clear what form or forms of synthesis will be possible with the available data. There are a number of different pathways from this point and therefore the following sections become more diverse in terms of the guidance given. They also become more reliant on guiding the reader to more detailed information sources.