Table 1 The Newcastle-Ottawa Scale (NOS) for assessing the quality of case controlled studies in meta-analyses

1) Is the case definition adequate?

1) Comparability of cases and controls on the basis of the design or analysis

1) Ascertainment of exposure

 a) yes, with independent validation*

 a) secure record (e.g.: surgical records)*

 b) yes, e.g. record linkage or based on self reports

 b) structured interview where blind to case/control status*

 c) no description

 a) study controls for __ (Select

2) Representativeness of the cases

the most important factor.)*

 c) interview not blinded to case/control status

 a) consecutive or obviously representative series

 b) study controls for any additional factor (This criteria could be modified to indicate specific control for a second important factor.)*

of cases*

 d) written self report or medical record only

 b) potential for selection biases or not stated

3) Selection of Controls

 e) no description

 a) community controls*

2) Same method of ascertainment for cases and controls

 b) hospital controls

 c) no description

 a) yes*

 b) no

4) Definition of Controls

3) Non-Response rate

 a) no history of disease (endpoint)*

 a) same rate for both groups*

 b) no description of source

 b) non respondents described

 c) rate different and no designation