Table 3.

Parsimonious Model of Sustained Smoking Abstinence for 6 Months among Participants Assigned to Deposit-based Incentives (n = 1,024)

	OR	95% CI	P Value	AIC Increase*
Pre/contemplators vs. preparators	0.39	0.22–0.64	<0.001	12.18
Smoking history, yr			<0.001	10.85
10–20 vs. <10	0.90	0.53–1.55
>20 vs. <10	0.26	0.12–0.56
Age (1-yr increase)	1.05	1.02–1.07	0.001	8.64
≥1 yr of college vs. less than college	1.86	1.16–3.05	0.011	4.78
Months failed to pay bills in last year (1-mo increase)	1.07	1.01–1.12	0.010	4.20
Spare money this week (1-unit increase; reference = very little; range, 0–12)	1.10	1.01–1.19	0.031	2.52
Natural log of Kirby discounting score (1-unit increase)	0.89	0.80–0.99	0.038	2.23
Base model (design variables)†
Intervention arm
Competitive vs. individual deposits	1.10	0.72–1.69	0.65
Household income			0.190
$80–100k vs. >100k	0.99	0.39–2.42
$60–80k vs. >100k	1.19	0.53–2.65
$40–60k vs. >100k	1.08	0.53–2.24
$30–40k vs. >100k	0.58	0.25–1.31
$20–30k vs. >100k	0.45	0.19–1.01
$10–20k vs. >100k	0.56	0.21–1.38
$10k vs. >100k	0.66	0.22–1.81
Has benefits vs. no benefits	1.17	0.75–1.80	0.49

Definition of abbreviations: AIC = Akaike information criterion; CI = confidence interval; OR = odds ratio.

^*The stepAIC procedure in R selects the best model based on the combination of variables that achieves the lowest AIC. The AIC for the best-fitting model was 643.45. The AIC shown in the last column indicates the absolute increase in the AIC (from 643.45 in the best model) if that single variable is removed from the full model shown in the table. Variables are presented in order of decreasing changes, indicating less predictive improvement related to that predictor.

^†The AIC for the base model, which included the three design variables (intervention arm, household income, and insurance benefits), was 682.46.