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Table 2 Grading of experimental, observational, and modelling studies according to the quality of evidence they provide to support causal associations between immunisation and its proposed benefits

From: The broader economic impact of vaccination: reviewing and appraising the strength of evidence

Quality of evidence Experimental studies Observational studies Modelling studies or conjecture
High Randomised trials Double-upgraded analytical observational studies  
Moderate Downgraded randomized trials Upgraded analytical observational studies  
Low Double-downgraded randomized trials Analytical observational studies  
Very low Triple-downgraded randomized trials Case series and case reports Modelling studies or conjecture