2.5 Statistical analysis
REGRESSION
The software IBM SPSS, version 27.0 (Chicago, Illinois, USA), was used for all statistical analyses and analyses were carried out non-blinded. The level of significance was set to 0.05. Baseline differences between groups were tested using independent sample t-test and Mann-Whitney U-Test.To test the effect of intervention outcomes explored over time, linear mixed-model regression analysis with repeated measures over the three time points, T0, T1, T2, was used with 2 levels: measurements nested within individuals. Mixed-model regression accounts for autocorrelations and models data that includes missing cases. Statistical assumptions were tested regarding normal distribution, multicollinearity, homoscedasticity, and linear relationship between variables. The most suitable model for exploring group differences in intervention effects was explored by building nested models and comparing each model to the previous one using −2 Log Likelihood values to determine improvement of model fit. First, a model which only included fixed effects of exposure and time and an interaction term (exposure*time) was built. Second, a random intercept on level 2 (individual) was added to the first model. Third, a random slope on level 2 was added. Fourth, a three-level model was tested without a random slope, thus where model 2 was nested, where prisons were added as level 3. Significant improvement of model fit was achieved between the first and second model, whereas no significant improvement was achieved in the third or fourth model when a random slope and a third level were added. Therefore, the model used in the analysis of intervention effects included fixed effects of exposure and time and an interaction term (exposure*time) with a random intercept on level 2 (individual). Linear mixed-model regression was used for all outcomes except for contact frequency between parent and child where the intervention effect on the dichotomised variable displaying contact at least once per week or less than once a week was explored using mixed-model logistic regression. The sample size did not allow for any moderation analyses. Analyses were undertaken using the intention-to-treat (ITT) approach where all parents who consented to participation and provided data at baseline were included. This analysis is reported in the tables and in text. In addition, an analysis per protocol, using the same model as in the ITT analysis but comparing parents in the intervention group with attendance in all 10 intervention sessions to all participants who provided data in the control group, was undertaken but is reported in text only.An a priori power calculation based on individuals and assuming equality between groups was conducted using the G*Power software [Process data regarding fidelity and dose were analysed using descriptive statistics and is reported as means (SD), and proportions in text.