Table 1

Quality ratings for comparison of different interventions

Comparison	Quality of direct evidence	Quality of indirect evidence	Quality of network meta-analysis evidence
Se vs placebo	Low*†	Low*‡	Low*†
Se salt vs placebo	Low*†	Low*‡	Low*†
Se+VC vs placebo	Moderate*	Low*‡	Moderate*
Se+VE vs placebo	Very low*†§	Low*‡	Low*‡
Se yeast vs placebo	Moderate*	Low*‡	Moderate*
VC vs placebo	Moderate*	Low*‡	Moderate*
Se salt vs Se	–	Very low*§¶	Very low*§¶
Se+VC vs Se	Moderate*	Low*§	Moderate*
Se+VE vs Se	–	Very low*§¶	Very low*§¶
Se yeast vs Se	Moderate*	Very low*§¶	Moderate*
VC vs Se	Moderate*	Low*¶	Moderate*
Se+VC vs Se salt	Low*‡	Low*¶	Low*¶
Se+VE vs Se salt	–	Very low*§¶	Very low*§¶
Se yeast vs Se salt	–	Very low*§¶	Very low*§¶
VC vs Se salt	Low*§	Very low*§¶	Low*,¶
Se+VE vs Se+VC	–	Very low*§¶	Very low*§¶
Se yeast vs Se+VC	–	Very low*§¶	Very low*§¶
VC vs Se+VC	Moderate*	Very low*§¶	Moderate*
Se yeast vs Se+VE	–	Very low*§¶	Very low*§¶
VC vs Se+VE	–	Very low*§¶	Very low*§¶
VC vs Se yeast	–	Very low*§¶	Very low*§¶

*Limitations (risk of bias).
†Inconsistency.
‡Inconsistency for indirect evidence: prediction intervals for treatment effect include effects that would have different interpretations.
¶Indirectness: no convincing evidence for the plausibility of the transitivity assumption.
§Imprecision.
Se, sodium selenite; Se salt, selenium salt; Se+VC, the combination of sodium selenite with vitamin C; Se+VE, the combination of sodium selenite with vitamin E; Se yeast, selenium enriched yeast; VC, vitamin C.