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Forest plot showing the effect of vitamin B12, zinc and iron on Mathematics, expressed as mean differences between the values obtained in the intervention and control groups. A positive effect size indicated that vitamin B12, zinc and iron increased Mathematics skills. Horizontal lines represent 95% CIs. Diamonds indicate the pooled-effect size from the random-effects analysis. The values ±0.2, ±0.5 and ±0.8 represent small, medium and large effect sizes, respectively [17,23].
