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Table 1 Assessment scale. Adapted from Dufault and Klar [29]

From: Mortality and morbidity in populations in the vicinity of coal mining: a systematic review

  Item Description
Study design and focus (max = 4) Sample size (max = 2) Number of ecologic units included in the analysis as a proportion of the total number of units (3 levels: < 11% = 0 points; 11-79% = 1 point; > 79% = 2 points)
Level of inference (max = 1) The results of the analysis are not used to draw inferences for individuals
Pre-specification of ecological units (max = 1) Ecological units are selected to suit the hypothesis (as opposed to seemingly motivated by convenience or necessity such as the use of districts, towns or counties)
Statistical methodology (max = 5) Validity of statistical inferences (max = 2) Number of ecological units per covariate (3 levels: 0-10 = 0 points; 10-20 = 1 point; > 20 = 2 points)
Use of covariates (max = 1) Analysis adjusted for covariates (e.g. sociodemographic; environmental risk factors)
Proper adjustment for covariates (max = 1) Covariates are properly adjusted when regressed upon adjusted outcomes as recommended for ecological studies [30]
Spatial effects (max = 1) Inclusion of spatial analysis
Quality of reporting (max = 3) Statement of study design (max = 1) Key elements of the study design are presented in the report
Justification of study design (max = 1) Justification of the ecological analysis, the rationale and the objectives are presented in the report
Discussion of cross-level bias and limitations (max = 1) Readers are cautioned about the limitations of the ecological design and/or the ecological fallacy