Table 9A Parametric variables used in classification and regression tree statistics (mean (SD)).
| Risk factors | Smokers | Non‐smokers |
|---|---|---|
| Age* | 16.4 (1.2) (n = 1257)† | 15.8 (1.2) (n = 2047) |
| Number of siblings | 3.5 (2.3) (n = 1222) | 3.1 (2.1) (n = 1993) |
| Mean grade of academic performance* | 3.6 (0.8) (n = 1088) | 3.9 (0.8) (n = 1768) |
| Mother's age* | 41.5 (6.1) (n = 1245) | 40.3 (5.6) (n = 2029) |
| Education time of mother* | 5.9 (4.4) (n = 1245) | 6.4 (4.2) (n = 2029) |
| Father's age* | 45.9 (6.3) (n = 1245) | 44.9 (6.2) (n = 1956) |
| Education time of father | 8.1 (4.0) (n = 1192) | 8.5 (3.9) (n = 1956) |
| Number of family members | 5.4 (2.4) (n = 1239) | 5.1 (1.9) (n = 2018) |
*Variables that are statistically significant splitters of this method. †The numbers in parentheses represented the numbers of subject analysed for each category of variables.