Table 2.

Customised checklist for risk of bias assessment

A	Quality of description of included geographical area or data sources
+	Clear description of geographical area and participating centres within the area or data sources. For example, data are presented for the number, sex, and age distribution of the population. Where relevant, providing complete referencing details to original data collection procedures or sources is adequate
?	Unclear or incomplete description of geographical area, its residents, and hospitals or data sources used
−	No description of investigated area
B	Quality of description of included population denominator
+	Clear description of inclusion and exclusion criteria is presented. Minimal criteria were considered sex, age, and (definition of) diabetes
?	Unclear or incomplete description of inclusion and exclusion criteria
−	No description of inclusion and exclusion criteria
C	Quality of description of diabetic foot disease or amputation numerator
	Diabetic foot disease	Amputation
+	Clear description of reported diabetic foot disease characteristics in line with a published classification system using objective measurement methods	Clear description of the levels of amputation in line with international guidelines or codes, and a clear definition of whether amputations were the first, recurrent, or any in sequence
?	Unclear or incomplete description of reported diabetic foot disease characteristics, or using subjective measurement methods	Unclear or incomplete description of levels of amputation in line with international guidelines or codes, or unclear or incomplete definition of sequence of amputation
−	No description of diabetic foot disease characteristics	No description of levels of amputation or sequence of amputation
D	Quality of information flow
+	Information flow was complete for each stage of the study Are the number of individuals at each stage of study reported—including number potentially eligible, number included, and number completed to follow-up?
?	Only national health statistics used, missing information for some stages or relevant time points
−	Poor or no description given on each stage and missing all important information
E	Quality of study design
+	Prospective study with diabetic foot disease or amputation as a predefined primary or secondary outcome
?	Retrospective study with diabetic foot disease or amputation as a primary outcome
−	Retrospective study with diabetic foot disease or amputation as a secondary outcome