To evaluate if Internet delivered emotion regulation individual therapy for adolescents is an efficacious treatment when delivered as adjunctive to treatment as usual compared to a control group consisting of treatment as usual.

Statistical Analyses

Treatment effects will be evaluated according to the intention-to-treat principle. Primary end-point for all outcomes is post-treatment. Secondary end-point include a controlled 3 month follow-up.

Primary Analysis

The primary outcome analysis will include treatment group (ERITA , TAU) and weekly reports of NSSI frequency measured once every week 4 weeks prior to treatment start, once every week during treatment, and once every week four weeks after treatment termination. Regression analysis modelled for count data will be used to estimate trend over time. Four weeks after treatment termination will be considered primary endpoint. Pairwise contrasts (group x time interaction) from the regression model will be used to evaluate between-group differences at primary endpoint (4 weeks after treatment termination).

Secondary Analyses

Secondary outcomes measured once before, during, and after treatment will be analyzed in a similar fashion as for the primary outcome, modelled after data's distribution (count or continuous). Measures collected only once at baseline and post-treatment will be analyzed with regression analyses including treatment group (online ERITA , TAU) as the between-subjects factor, time (baseline to post) as the within-subjects factor, and group × time interactions, modelled after data's distribution (count or continuous).

Parallel process latent growth curve modeling will be used to determine whether change in week-to-week emotion dysregulation (DERS-16) during treatment mediate the overall effect of ERITA+ETAU on week-to-week change in the primary outcome self-rated NSSI frequency (DSHI-Y).

3 Month Follow-Up

Data collected at 3-month follow-up will be used to determine the extent to which potential treatment gains were maintained both within and between treatment conditions

Update 28 April 2022: As a secondary analyses, we also aim to investigate the effect of treatment moderators (primary aim) and predictors (secondary aim) on NSSI frequency one month after treatment termination. This update was submitted prior to conducting the analyses.

The selection of potential moderators was theoretically and empirically motivated. We will investigate the following variables (measured at baseline):

Age Difficulties in emotion regulation As measured with the Difficulties in Emotion Regulation Scale - 36 item version (DERS) Past month NSSI frequency As measured with the Deliberate Self-Harm Inventory Youth version (DSHI-Y) Global functioning As measured with the Childrens Global Assessment Scale Depressive symptoms As measured with the 21-item Depression Anxiety Stress Scales (DASS-21) Sleep difficulties As measured with the Insomnia Severity Index (ISI) Parental Minimization - discounting or devaluing the child's negative emotions/problems As measured with The Coping with Children's Negative Emotions Scale Adolescent Version (CCNES-A), subscale minimazation Level of suicidality As measured with the MINI-KID International Neuropsychiatric Interview, version 6

Inclusion Criteria:

• ≥ 5 nonsuicidal self-injury episodes past year
• ≥ 1 nonsuicidal self-injury episodes past month
• having at least one parent who committed to participate in the parent program

Exclusion Criteria:

• Severe suicidal ideation
• a diagnosis of psychotic or bipolar I disorder or ongoing (past month) substance dependence
• the presence of co-occurring psychological disorders that required immediate treatment (i.e., severe anorexia nervosa)
• insufficient understanding of the Swedish language