Table 2. Differences in Kruskal-Wallis statistics to identify intergroup differences between the samples for BMI according to chronological age and BMI according to height-age in boys.
| Age (years) | Kruskal-Wallis statistic BMI-for-chronological age | Kruskal-Wallis statistic BMI-for-height-age | Difference between Kruskal-Wallis statistics |
|---|---|---|---|
| 1 | 3.77 | 4.99 | -1.22 |
| 2 | 1.79 | 1.97 | -0.18 |
| 3 | 8.71 | 13.04 | -4.33 |
| 4 | 1.04 | 8.04 | -7.00 |
| 5 | 11.08 | 3.33 | 7.75 |
| 6 | 8.54 | 3.92 | 4.62 |
| 7 | 5.31 | 2.10 | 3.21 |
| 8 | 16.21 | 2.03 | 14.18 |
| 9 | 10.38 | 3.91 | 6.47 |
| 10 | 7.63 | 0.89 | 6.74 |
| 11 | 9.51 | 0.37 | 9.14 |
| 12 | 1.79 | 4.66 | -2.87 |
| 13 | 22.63 | 1.06 | 21.57 |
| 14 | 2.11 | 1.63 | 0.48 |
| 15 | 0.07 | 1.69 | -1.62 |
| 16 | 5.90 | 0.20 | 5.70 |
| 17 | 2.94 | 2.73 | 0.21 |
| Total | 62.85* |
*
P=0.019
To test whether the median BMI differed according to height-SDS we calculated the Kruskal-Wallis statistics (KW). In order to examine whether BMI-for-age would result in more similar BMI distributions than BMI-for-height-age, per age group differences between the KWs were calculated and summed. Subsequently, a permutation test was used to test whether these differences were statistically significant. A positive difference in KW means that height related BMI differences were larger for BMI-for-age as compared to BMI-for-height-age.