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Table 1 Bias assessment tool

From: Methodological limitations in studies assessing the effects of environmental and socioeconomic variables on the spread of COVID-19: a systematic review

Bias domain Question to Consider Indicator Score
(1: Low Risk of Bias; 2: Medium Risk; 3: High risk)
Confounding bias Did the study analysis adjust potential confounders appropriately? Confounders adjusted 1: Many confounders and unobserved confounding
2: Some confounders (in particular mobility or socioeconomic variables) or unobserved confounders
3: None or cannot tell
Control of the population Did the study control for population? Population, age structure of the population 1: Control and/or including population density
2: Control only by including population density
3: No control
Control of the spatial and/or temporal dependence Did the study control the spatial and/or temporal extra variability? Spatial and temporal dependence 1: Control
2: Partial control (control of only one dependence, for example)
3: No control
Control of non-linearities Did the study control for non-linearities? Non-linearities (parametric or non-parametric) 1: Control
3: No control
Measurement errors What was the heterogeneity of indicators used in the study? Measurement errors in the explanatory variables (exposure variables and covariates) 1: Control
2: Partial control (including lags, for example)
3: No control
Statistical model Did the study use appropriate statistical model? Statistical model 1: Models for count data response variables
2: Control of heteroscedasticity and rates as response variables
3: Models with normally distributed errors and count data response variable; or no control of heteroscedasticity and rates as response variables
Overall Study Rating Strong (low risk of bias): one domain, at most, was rated as 3
Moderate: up to 2 domains were rated as 3
Weak (high risk of bias): ≥ 3 domains were rated as 3