Table 1 Check list for quality assessment and scoring of studies based on NOS

Check list

Selection

 1. Representativeness of the sample. Truly representative or somewhat representative? (if yes, one star)

 2. Sample size ≥ 40 (if yes, one star)

 3. How representative was the C2 pedicle group in comparison with C2 pars screw placement in upper cervical patients, and the accuracy rate assessment is satisfactory? (if yes, one star; no star if the patients were selected only in one group)

 4. Ascertainment of the risk factors as surgical record: Were the risk factors measured with valid and reliable instruments? (if yes, one star)

Comparability

 The accuracy rate screw placement and any additional factors as age, gender, and accurate classification of radiological malposition in different outcome groups are comparable, based on the study design or analysis. Confounding factors are controlled. (if yes, two stars; one star was assigned if one any additional factors was not reported)

Outcome assessment

 6. Ascertainment of the outcome: clearly defined outcome of accuracy rate (yes, two star for information ascertained by record accuracy rate based on classification of radiological malposition; one star if this information was not reported)

 7. Appropriate statistical analysis: The statistical test used to analyze the accuracy rate is clearly described and appropriate for C2 pedicle or pars pedicle (if yes, one star; no star was assigned if the accuracy rate is reported overall)