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Table 1 Classification of study designs

From: Quality assessment tools for evidence from environmental science

Study design Description
Randomised Controlled Trial (RCT) RCTs are studies in which study units are randomly allocated to intervention or control groups and followed up over time to assess any differences in outcome rates. Randomisation with allocation concealment (double blinded) ensures that on average known and unknown determinants of outcome (including confounding factors) are evenly distributed between groups.
Observational Study Observational studies are studies in which natural variation in interventions (or exposure) among study units is investigated to explore the effect of the interventions (or exposure) on outcomes. These include designs such as:
• Controlled before-and-after study: A follow-up study of study units that have received an intervention and those that have not, measuring the outcome variable both at baseline and after the intervention period, comparing either final values if the groups are comparable at baseline, or change in scores if they were not.
• Concurrent cohort study: A follow-up study that compares outcomes between study units that have received an intervention and those that have not. Study units are studied during the same (concurrent) period either prospectively or, more commonly, retrospectively.
• Historical cohort study: A variation on the traditional cohort study where the outcome from a new intervention is established for study units studied in one period and compared with those that did not receive the intervention in a previous period, i.e. study units are not studied concurrently.
• Case–control study: Study units with and without a given outcome are identified (cases and controls respectively) and exposure to a given intervention(s) between the two groups compared.
• Before-and-after study: Comparison of outcomes from study units before and after an intervention is introduced. The before and after measurements may be made in the same study units, or in different samples.
• Cross-sectional study: Examination of the relationship between outcome and other variables of interest as they exist in a defined population at one particular time point.
Case Study Case studies are studies which describe a number of cases of an intervention and outcome, with no comparison against a control group.