Table 2.

Rules for classifying variables regarding strength of association

Outcome measures supporting association (%)	Summary code	Explanation of code
0–33	0	Non-significant association
34–59	?	Inconclusive association
60–100	+	Positive association
60–100	-	Negative association

Note: When a correlation was observed in three or more studies, it was coded as: 00 (non-significant association for three or more studies); ?? (inconclusive for three or more studies); +  + (positive association for three or more studies); – (negative association for three or more studies). This assists visually with correlations that were more widely studied