Table 1 Defining the strength of a risk factor

Definite

 All low and moderate risk of bias studies positive (at least three studies)

 Majority (more than 50%) low and moderate risk of bias studies positive (at least five studies)

Likely

 All low and moderate risk of bias studies positive (two studies)

 Majority (more than 50%) low and moderate risk of bias studies positive (2–4 studies)

Unclear

 All low and moderate risk of bias studies positive (one study)

 Low and moderate risk of bias studies show mixed or conflicting results

 A majority (more than 50%) of studies negative but at least one low or moderate risk of bias study positive

Not a risk factor

 No low or moderate risk of bias studies positive