Table 3.
Summary of model building process for multilevel mediation model.
| Model | Hypothesis/Rationale for Step | Results | AIC | BIC | vs. | χ2 | df | p |
|---|---|---|---|---|---|---|---|---|
| 1. Base model. Indirect associations from subjective response variables to BrAC mediated via craving at both within- and between-person levels. No direct paths from stimulation and sedation to BrAC. No random slopes. | At the within-person level, subjective responses will be associated with craving, and craving will be associated with BrAC. | Hypothesized associations observed. | 4938.81 | 4948.52 | - | - | - | - |
| 2. Add direct paths from stimulation and sedation to BrAC at both levels. | No hypothesis; Data driven step of determining partial or full mediation. | Inclusion of direct paths significantly improved model fit. | 4905.69 | 4916.50 | 1 | 46.22** | 4 | <.01 |
| 3. Add random slope for within-person association between craving and BrAC. | Random slope will have significant variance (which will then be predicted by impaired control in step 5). | Inclusion of random slope significantly improved model fit. | 4905.30 | 4917.50 | 2 | 12.98* | 5 | .02 |
| 4. Add random slopes for other within-person associations: | No hypothesis; Data driven step of determining appropriate random effects structure. | Random slope for stimulation to craving should be included. | ||||||
| a. stimulation to craving | 4901.66 | 4915.52 | 3 | 14.13* | 6 | .03 | ||
| b. sedation to craving | 4907.62 | 4921.48 | 3 | 6.72 | 6 | .35 | ||
| c. stimulation to BrAC | 4902.54 | 4916.41 | 3 | 8.73 | 6 | .19 | ||
| d. sedation to BrAC | 4906.82 | 4920.69 | 3 | 0.74 | 6 | .99 | ||
| 5. Add paths from impaired control, drinks per drinking day, and impulsivity to random intercepts and random slope for the within-person association between craving and BrAC. | Impaired control will be independently associated with the random slope (i.e., a significant moderator of the association) while controlling for alcohol use and trait impulsivity; paths to random intercepts are necessary to control for main effects before examining cross-level interaction. | Significant paths from impaired control to random slope observed; No paths from drinks per drinking day or trait impulsivity were significant. | 4902.35 | 4918.71 | 4a | 17.96* | 9 | .04 |
| 6. Trim nonsignificant paths from between-person covariates to random slope and intercepts. | No hypothesis; Data-driven step of trimming model of unnecessary parameters to achieve best-fitting model. | Model fit the data just as well as model 5 and is more parsimonious. | 4894.13 | 4908.82 | 5 | 3.61 | 6 | .73 |
Note. Model 6 represents the final model. AIC= Akaikeinformation criterion; BIC = Bayesian information criterion (sample size adjusted); vs. = comparison model for likelihood ratio test; χ2 value is the test statistic for the likelihood ratio test.
*
p≤.05;
**
p≤.01.