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Table 1 Overview of the broad types of study included in comparative environmental systematic reviews addressing PECO/PICO-type questions (for detail see Additional file 3)

From: Principles and framework for assessing the risk of bias for studies included in comparative quantitative environmental systematic reviews

Experimental studies Randomised controlled experiments e.g. before-after-control-impact (BACI) studies, control-impact studies, before-after (BA) studies
Non-randomized controlled experiments
Observational studies Case–control studies e.g. non-intervention spatial comparison studies
Temporal monitoring studies e.g. prospective or retrospective time series studies
Cross-sectional studies e.g. surveys, questionnaires, interviews; studies of spatial occurrence, distribution or prevalence (may be embedded within other study designs)
Other study types Combined experimental and observational studies Wide range of designs possible