Linear regression with geometric mean cotinine concentration as outcome variable, adjusting for age, number of parents who smoke and interaction terms between year and family socioeconomic classification (SEC)* (model 1, n=3322) and year and family affluence scale (FAS) (model 2, n=4049)
| Model 1 | ||||
| Variable | β | 95% CI | Wald test p value | |
| Year (ref: 2006) | 2007 | −0.56 | (−0.73 to −0.38) | <0.001 |
| Family SEC (ref: SEC 1) | SEC 2 | 0.32 | (0.17 to 0.47) | <0.001 |
| SEC 3 | 0.45 | (0.26 to 0.65) | ||
| SEC 4 | 0.82 | (0.60, 1.04) | ||
| Family SEC*year interaction (ref: SEC 1*2006) | SEC 2*2007 | −0.05 | (−0.26 to 0.16) | 0.278 |
| SEC 3*2007 | 0.17 | (−0.16 to 0.50) | ||
| SEC 4*2007 | 0.17 | (−0.13 to 0.46) | ||
| Model 2 | ||||
| Year (ref: 2006) | 2007 | −0.61 | (−0.77 to −0.45) | <0.001 |
| Family affluence scale (FAS) (ref: high FAS) | Medium FAS | 0.15 | (0.01 to 0.29) | <0.001 |
| Low FAS | 0.41 | (0.27 to 0.55) | ||
| FAS*Year interaction (ref: High FAS*2006) | Medium FAS*2007 | 0.03 | (−0.18 to 0.24) | 0.074 |
| Low FAS*2007 | 0.23 | (0.01 to 0.46) |
↵* Family socioeconomic classification: family SEC1=I professional occupations & II managerial and technical; SEC2=IIIN skilled non-manual & IIIM skilled manual; SEC3=IV partly skilled and V unskilled; SEC4=economically inactive.