Table 1. Quality scoring criteria.

Evaluating items	Quality Criteria	Quality Score
1. Selection		0–4
1.1 Is the case definition adequate?	Requires some independent validation (e.g. >1 person/record/time/process to extract information, or reference to primary record source such as pathology or medical/hospital records)  = 1
	Record linkage (e.g. International Classification of Diseases ICD codes in database) or self-report with no reference to primary record, or no description  = 0
1.2 Representativeness of the cases	consecutive or obviously representative series of cases  = 1All eligible cases with outcome of interest over a defined period of time, all cases in a defined catchment area, all cases in a defined hospital or clinic, group of hospitals, health maintenance organisation, or an appropriate sample of those cases (e.g. random sample)
	Not satisfying requirements above, or not stated  = 0
1.3 Selection of Controls	community controls (i.e. same community as cases and would be cases if had outcome) = 1
	hospital controls, within same community as cases (i.e. not another city) but derived from a hospitalized population or no description  = 0
1.4 Definition of Controls	no history of disease (endpoint)  = 1If cases are first occurrence of outcome, then it must explicitly state that controls have no history of this outcome. If cases have new (not necessarily first) occurrence of outcome, then controls with previous occurrences of outcome of interest should not be excluded.
	no description of source  = 0
2. Comparability		0–2
1) Comparability of cases and controls on the basis of the design or analysis	study controls for age and region  = 1
	study no controls for age and region  = 0
	study controls for heredity factors  = 1
	study no controls for heredity factors  = 0
3. Exposure		0–4
3.1 Ascertainment of exposure	assessment of exposure by measurement or strict caculation  = 1
	assessment of exposure by questionnaire or no description  = 0
	assessment of exposure including environmental, living and occupational exposure  = 1
	assessment of exposure only including one aspect of life or work such as usage of electric heating equipment, work duty and distance from the high voltage power lines  = 0
3.2 Same method of ascertainment for cases and controls	Yes  = 1
	No  = 0
3.3 Non-Response rate	same rate for both groups  = 1
	non respondents described or rate different and no designation  = 0
Total score		0–10