Forest plot for the effect of vitamin A treatment on measles mortality for at least two doses of 200 000 IU for children and 100 000 IU for infants (treated vs untreated). Heterogeneity χ2 = 1.05 (df = 2); P = 0.592. I2 (variation in RR attributable to heterogeneity) = 0.0%. Test of RR = 1: z = 2.50; P = 0.01