Boxplots for intrusive memory (IM) measures.
The midline of the boxplot is the median value, with the upper and lower limits of the box being the third and first quartile (75th and 25th percentile), and the whiskers covering 1.5 times the IQR. The dots depict outliers (each dot represents one participant that departed more than 1.5 times the IQR above the third quartile and below the first quartile). All outliers are included in this figure. In Bayesian hypothesis testing, a metric known as a Bayes Factor (BF) [In contrast to For this study, BFs were computed repeatedly during interim analyses, starting with 20 participants and approximately every 4–10 participants thereafter, up to a maximum of 150. Early stopping of the trial for either sufficient evidence of negative or positive effect was considered if the respective BFs exceeded a pre-defined threshold of 20 which would suggest strong evidence [We first calculated a BF for a negative effect of the intervention (i.e. whether participants in the immediate arm had a greater number of IMs at week 4 than the delayed arm). If this BF exceeded 20, we concluded that there was strong evidence for a negative effect of the intervention and the trial may need to be altered or stopped. We then calculated a BF for positive treatment effect (i.e. whether those in the immediate arm had fewer IMs at week 4 than the delayed arm, as opposed to having no difference). If this BF exceeded 20, we concluded that there was strong evidence for the effectiveness of the intervention and consideration could be given to stopping the trial early (Supplementary Information).Originally, we planned to explore potential “mechanistic” optimisations to improve the effectiveness of the intervention (e.g., time playing Tetris®). Given the rapid accumulation of evidence in favour of a positive treatment effect (see “Results”), we focused on practical “usability enhancements” to aid smooth digital delivery and user experience (Supplementary Information): this included repeating the intrusive memory visualisation step, adding a summary instruction video, and adding a graphical representation of daily IMs for the 4 weeks. An optimisation enhancement round was conducted on Feb 7, 2022, after 55 participants had been randomised. When testing for the effect of these enhancements, sample size analyses were first conducted to estimate the number of participants needed to test for a positive treatment effect, and to compare pre-and post-optimisation groups (Supplementary Figs. 