A controlled pre-post design study on Take it Personal! has demonstrated effectiveness in reducing the frequency and severity of youth use of alcohol, cannabis or other illicit drugs. Take it Personal! is an existing indicated prevention programme for substance use in youth with a mild intellectual disability or borderline intellectual functioning that addresses each participant's high-risk personality traits for substance abuse. The current Take it Personal! programme is further developed and optimized in collaboration with relevant stakeholders. In particular, the investigators aim to integrate personalized daily diary monitoring in the programme so that trainers can monitor client progresses closely and gain insights into change mechanisms, providing starting points for therapeutic efforts in programme sessions. The investigators conduct a series of case studies with a non-concurrent multiple baseline design to evaluate the effectiveness of Take it Personal!. The baseline lengths are randomly determined, and therefore the start of the intervention is staggered across participants.

Procedure Participants have either 14, 17, 20 or 23 days of baseline measurements. After baseline, participants will follow Take it Personal! for six weeks. Lastly, there is a follow-up phase in which each participant completes 30 days of daily diaries.

The Ethica mobile phone application is used for data collection, which facilitates user-friendly editing and adding of items to optimally personalize monitoring and care. Trainer and client can translate intervention goals into the participant's daily diary items. Completing daily diaries primarily has a clinical incentive for participants, as responses are discussed in Take it Personal!. That is, during the programme, data are fed back to the therapists to facilitate deeper knowledge on participant's behavioural and affective patters. In individual sessions, participant and trainer will inspect and interpret the daily monitoring data to gain more structured insights into changes over time.

To gain additional insight into the long-term effects on substance use frequency, problems caused by substance use and symptoms of dependence, the investigators administer the Alcohol Use Disorders Identification Test (AUDIT) and the Drug Use Disorders Identification Test (DUDIT), which are incorporated as subscales in the Substance Use and Misuse among Intellectually Disabled Persons Questionnaire (SumID-Q) and this survey's collateral report version (SumID-CR) to respectively participants and daily carers at baseline, 1-, 6- and 12-month follow-up.

The investigators add complementary qualitative components that will enable better interpretation of why changes occur or not occur for individuals. One month after the last Take it Personal! session, the Client Change Interview (CCI) is administered to the participant and his/her daily carer separately. The CCI is a semi structured interview, previously used in a mild intellectual disability, in which participants reflect on changes they did and did not notice since starting the programme and contemplate on what caused this. During the CCI, participant and the participant's primary carer are asked to reflect on during which periods changes occurred.

Sample size 15 participants

Sample size justification The joint Dutch quality assessment procedure ('erkenningstraject effectieve interventies') states that a series of at least 10 well-executed case studies, under different conditions and different therapists, is recognized to demonstrate strong evidence for intervention and prevention effectiveness. The investigators obtain an additional 30% to account for withdrawal, drop-out, missing data, and decreasing adherence rates, resulting in a required sample size of 15 participants who will enroll in a non-concurrent multiple baseline study with different therapists per group.

Inclusion criteria:

• Between 14 and 30 years of age
• A DSM-5 based diagnosis of mild intellectual disability OR borderline intellectual functioning A mild intellectual disability is characterized by an intelligence quotient (IQ) score between 50-70 and limitations in adaptive behavior that impede a range of everyday social and practical skills. The DSM-5 describes borderline intellectual functioning as a condition in which a person's limited intellectual functioning is the focus of, or has an impact on, their treatment. This diagnosis is typically given when IQ is roughly between 70 and 85. Persons with either diagnosis often lead problematic lives, facing, for example, social and coping difficulties, and are vulnerable to the development of psychopathologies such as substance use disorder.
• Receives specialized in- or outpatient care
• Uses alcohol or drugs at least once per two weeks, as confirmed by participant's clinician
• Owns a mobile phone

Exclusion criterium

- Moderate or severe substance use disorder

Preregistration, metadata and analyses scripts/syntax will be stored in the Open Science Framework (OSF; www.osf.io) and will be made findable and accessible publicly. The raw data (i.e., pseudonymized quantitative data in .csv of daily diaries, standardized surveys and anonymized transcripts of the client change interviews), as well the metadata, will be made findable and openly accessible on the repository of Radboud University: Radboud Repository. A persistent identifier will be made through OSF, which is then also included in the Radboud Repository. Hence, we use only one DOI. Sensitive data cannot be made publicly accessible. Therefore, audio recordings of the interviews nor any contact information (names, e-mail addresses, addresses) are not accessible.