Results from mixed effects negative binomial regression examining changes in canteen spend pre-implementation vs post implementation of smoke-free prison policy
| Variable | Total spend (£) | Nicotine-related products (£) | Food items (£) | |
| IRR (99% CI) | IRR (99% CI) | IRR (99% CI) | ||
| Full sample n=11 944 (group 1) | Prechange vs postchange | 0.92 (0.84 to 1.00) | 0.40 (0.33 to 0.51)† | 1.12 (0.99 to 1.27) |
| Change in time trend | 1.00 (1.00 to 1.00) | 1.02 (1.01 to 1.03)† | 1.00 (1.00 to 1.01) | |
| Pretime trend | 1.00 (1.00 to 1.00) | 0.99 (0.99 to 1.00) | 1.00 (0.99 to 1.00) | |
| ‘Smokers’ resident within a prison for more than 31 weeks (periods 1–3) n=2541 (group 2) | Prechange vs postchange | 0.91 (0.81 to 1.03) | 0.35 (0.25 to 0.50)† | 1.24 (1.08 to 1.43)* |
| Change in time trend | 1.00 (1.00 to 1.01) | 1.02 (1.01 to 1.04)† | 1.00 (0.99 to 1.00) | |
| Pretime trend | 1.00 (1.00 to 1.00) | 0.99 (0.99 to 1.00) | 1.00 (1.00 to 1.00) | |
| ‘Non-smokers’ resident within a prison for more than 31 weeks n=342 (group 3) | Prechange vs postchange | 0.87 (0.66 to 1.16) | 0.08 (0.00 to 1.62) | 0.80 (0.540 to 1.19) |
| Change in time trend | 1.01 (0.99 to 1.02) | 1.18 (1.03 to 1.36) | 1.01 (0.99 to 1.02) | |
| Pretime trend | 1.00 (0.99 to 1.01) | 1.03 (0.93 to 1.14) | 1.00 (0.99 to 1.01) |
*p<0.01
†p<0.001
IRR, incident rate ratio.