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Table 3 Vitamin C and asthma symptoms: effect modification by age and baseline asthma severity

From: Vitamin C and asthma in children: modification of the effect by age, exposure to dampness and the severity of asthma

Age (yr) C-ACT at baseline No of. Children Symptoms (C-ACT) Mean Arithmetic increase in C-ACTa) Percentage increment in C-ACTb)
    Placebo Vitamin C Mean 95% CI Mean 95% CI
7.0-8.2 18-19 12 17.6 21.8 4.2 3.3-5.2 24% 18-31%
  16-17 10 16.3 20.0 3.7 2.4-5.0 23% 15-32%
  13-15 8 15.5 18.5 3.0 2.4-3.6 19% 16-23%
8.3-10 18-19 8 17.2 21.0 3.7 1.6-5.8 23% 9-36%
  16-17 10 16.8 19.2 2.4 1.5-3.3 14% 9-20%
  14-15 12 15.8 17.2 1.3 0.1-2.5 9% 1-17%
  1. a) Arithmetic increase in the C-ACT. When age and baseline C-ACT as continuous variables were included in the linear model explaining the effect of vitamin C, the model was improved by χ2(2 df) = 19.1, P = 0.00007 (model parameters are shown in Table 4). There was no second order interaction between the baseline C-ACT and age in their modification of the vitamin C effect (χ2(1 df) = 2.1, P = 0.2).
  2. b) Percentage increment in the C-ACT. When age and baseline C-ACT as continuous variables were included in the linear model explaining the effect of vitamin C, the model was improved by χ2(2 df) = 14.0, P = 0.0009. There was no second order interaction between the baseline C-ACT and age in their modification of the vitamin C effect (χ2(1 df) = 2.6, P = 0.1).