Table 3.
Risk of Bias of ecological studies
| Study | Selection |
Comparability | Outcome |
|||||
|---|---|---|---|---|---|---|---|---|
| Exposure group representativea | Ascertainment of exposureb | Population exposedc | Comparable groupsd | Controls for confounderse | Assessment of outcomef | Appropriate time lagg | Statistical testh | |
| Mitze et al. [9] | * | – | * | * | ** | * | ** | * |
| Lan et al. [19] | * | – | * | – | – | * | * | * |
| Chernozhukov et al. [20] | * | – | – | * | ** | * | * | * |
| Chiang et al. [21] | * | – | – | – | – | * | – | – |
| Cheng et al. [22] | * | ** | – | * | – | – | – | – |
| Bo et al. [23] | * | – | – | – | ** | * | ** | * |
| Lyu and Wehby [17] | * | – | * | – | ** | * | ** | * |
| Gallaway et al. [24] | * | – | – | – | – | * | – | – |
| Leffler et al. [25] | * | – | – | – | * | * | – | * |
| Van Dyke et al. [26] | * | – | – | – | – | * | – | * |
| Kanu et al. [27] | * | – | – | – | – | * | – | – |
| Zhang et al. [28] | * | – | – | – | – | * | – | – |
| Zhang and Warner [33] | * | – | – | * | * | * | ** | – |
| Rader et al. [29] | * | ** | * | * | ** | * | ** | * |
| Li et al. [30] | * | – | – | * | – | * | ** | * |
| Rebeiro et al. [31] | * | – | – | – | * | – | * | * |
| Krishnamachari et al. [32] | * | – | – | – | – | * | – | * |
| Joo et al. [37] | * | – | * | * | * | * | ** | * |
| Dasgupta et al. [34] | * | – | – | * | ** | * | – | * |
| Guy et al. [35] | * | – | * | * | ** | * | * | * |
| Poppe [36] | * | – | – | * | – | * | * | * |
*Satisfactory.
** Good.
If studies chose a sample which were truly or somewhat representative of the average in the target population, we assigned 1 star.
If rate of mask wearing was assessed within the population, we assigned 1 star. If this included a measure of level of compliance, we assigned 2 stars.
If details about where masks should be worn and by whom, we assigned 1 star.
Where a comparison was made, the comparison group was appropriate (ie similar risk of outcome) or statistical adjustments were made, we assigned 1 star.
If other policy-level factors were controlled for (such as physical distancing, stay at home order, closure of public venues, restriction of gatherings), we assigned 1 star. If community level factors were controlled for (such as community prevalence and population size) we assigned 2 stars.
If study used case data corresponding to the target population, we applied 1 star.
If an appropriate lag time was incorporated to account for timing of effects of mask introduction and assessment outcome, we assigned 1 star. If a sensitivity analysis was conducted using a range of time lags, we assigned 2 stars.
If the statistical tests used to analyze the data was clearly described and appropriate, and the measurement of the association was presented with confidence intervals, we assigned 1 star.