Evaluating the Mentalization-based Treatment Lighthouse Parenting Programme: Protocol of a Feasibility and Acceptability Quasi-Experimental Study

Feasibility of the recruitment strategy will be assessed based on the participant enrollment rate, that is the ratio of acceptance to enter the study by the number of service users invited. This will be tracked through recording the number of service users approached for recruitment and responses to invitations (whether accepting or not), including reasons for rejecting assessed through qualitative feedback from participants. Data will be systematically registered in Excel spreadsheets.

Sensitivity to change of clinical outcome measures will be evaluated based on the analysis of within-group change over time, particularly the direction and magnitude (e.g., effect sizes, mean differences) of observed changes in relation to intervention targets. This assesses whether measures can detect expected change.

Distributional properties of clinical outcome measures will be evaluated based on descriptive statistics of scores, including score distributions, variability (e.g., SD, range), floor effects and ceiling effects. Results will reflect how appropriately each measure behaves in the sample.

Reliability of clinical outcome measures will be evaluated based on the internal consistency coefficients (e.g., Cronbach's alpha) of scales within the study sample.

Construct coherence of clinical outcome measures will be evaluated based on correlations between theoretically related constructs. This evaluates whether measures behave as expected theoretically.

Completion rates of clinical outcome measures will be evaluated by calculating the percentage of participants that completed the psychological assessments at each time point. This reflects procedural feasibility of data collection.

Missing data on clinical outcome measures will be evaluated based on missing data rates (percentage of missing data) and patterns (e.g., systematic vs random). This allows to explore the adequacy of clinical outcome measures and, if missing data is substancial and not at random, the factors that may be affecting measure suitability.

Fidelity to treatment (experimental condition only) will be assessed by a supervisor-informed rating system, specifically developed for this study and adapted from the existing tool used at the ERiC trial (Nick Midgley, Holly Dwyer Hall and Emma Morris, Anna Freud, unpublished 2024), evaluating the quality of the therapists' mentalizing stance on four different dimensions pertaining to key MBT principles, each rated on a 4-point Likert scale, assessed across supervision sessions. Scores ≥3 per dimension are considered satisfactory. Fidelity to treatment will also be evaluated based on the percentage of key treatment activities (experiential and psychoeducational) that facilitators were able to deliver - these will be tracked by facilitators' self-report of activities conducted in each session.

Attendance rates will be calculated based on the mean number of sessions attended by participants; retention rates will be calculated by the percentage of participants that have not dropped-out across different timepoints of the study; reasons for discontinuation will be assessed by qualitative feedback from participants; this data will be registered in Excel spreadsheets.

Acceptability of the Lighthouse Parenting Programme from participants' perspective will be assessed through semi-structured interviews with participants in the experimental condition focused on their experiences of the programme, their views on the intervention's appropriateness (cultural, clinical, contextual), and potential barriers or facilitators to attending and engaging with the programme

Acceptability of the Lighthouse Parenting Programme from the facilitators' perspectives will be assessed through focus groups with facilitator dyads, focusing on their perceptions of the appropriateness of the intervention, barriers and facilitators to participant attendance and to the delivery of the programme

Acceptability of study procedures will be assessed through:

• Semi-structured interviews, exploring: participants' perspectives on the adequacy of study's procedures, including recruitment strategy and data collection measures (e.g., burden, clarity and relevance).

Interviews will be conducted at post-test, given the risk of missing data at follow-up assessments. Participants who attend follow-up assessments will also respond to an additional interview question regarding the acceptability of the follow-up assessment procedures. Both retained participants and those who drop out will be invited to be interviewed. For participants who formally withdraw from the study, the timing of the interview will remain flexible, with priority given to conducting it as soon as possible after withdrawal.

Acceptability of study procedures will be assessed through focus groups with facilitator dyads, focusing on adequacy e replicability of study's procedures

Sociodemographic data will be collected, including: age and sex of participant, marital status, level of education, employment status, family income, number of children, age and sex of children, measure of Child Protection applied and duration.

Parental history of adverse childhood experiences will be evaluated through the Adverse Childhood Experiences questionnaire (ACE; Felitti et al.1998; Portuguese version by Silva & Maia, 2008). The ACE assesses exposure to potentially traumatic experiences in childhood, such as abuse, neglect, and household adversity (29 items). Includes the following subscales: 1) Emotional abuse, 2) Physical abuse, 3) Sexual abuse, 4) Emotional neglect, 5) Physical neglect, 6) Divorce or parental separation, 7) Exposition to domestic violence, 8) Substance abuse in the family, 9) Mental health or suicide, 10) Incarceration of family member.

Parental Reflective Functioning will be evaluated through the current gold standard measure Parent Development Interview - Reflective Functioning (PDI-RF; Slade et al., 2004). The PDI is an interview-based measure of parental reflective functioning, assessing the parent's capacity to understand their own and their child's mental states. The data analyst is certified by the PDI Institute (https://www.pditraininginstitute.com/) to code the PDI.

Child's psychological adjustment will be assessed with the parent-report version of the Strengths and Difficulties Questionnaire (SDQ; Goodman et al., 1997, 1999; Portuguese version available at http://www.sdqinfo.com), a brief 25-item screening measure of children's emotional and behavioural symptoms as per reported by parents. The SDQ includes the following subscales: 1) Total Difficulties, 2) Emotional Symptoms, 3) Conduct Problems, 4) Inattention/ Hyperactivity, 5) Peer-Problems, and 6) Prosocial behaviour.

Parenting stress will be evaluated with the Parenting Stress Scale (PSS; Berry & Jones, 1995; Portuguese version by Algarvio et al., 2018), a 14-item measure to assess perceived stress associated with parenting, balancing parenting demands with rewards and satisfaction. The PSS includes the following subscales: 1) Parental stressors, 2) Parental satisfaction, 3) Lack of control, and 4) Fears and anxieties.

Parent cognitions will be assessed with the Parent Cognition Scale (PCS; Snarr et al., 2009; Portuguese version by Fernandes et al., 2019), a 30-item measure to assess parents' beliefs, interpretations, and attributions about their child's misbehaviour. The PCS includes two subscales: 1) Child-responsible attributions, and 2) Parent-causal attributions.

As a theoretical mediator of change, this outcome will be assessed at two additional timepoints mid-treatment.

Parental emotion regulation difficulties will be assessed with the Difficulties in Emotion Regulation Scale - Short Form (DERS-SF; Kaufman et al., 2016; Portuguese version by Gouveia et al., 2018). The DERS-SF is a 18-item screening tool that measures difficulties in understanding, accepting, and managing emotions, including impulse control and emotion regulation strategies. It includes the following subscales: 1) Strategies, 2) Goals, 3) Impulse, 4) Clarity, 5) Awareness, 6) Non-Acceptance, and 7) Difficulties in emotion regulation [Total Score].

As a theoretical mediator of change, this outcome will be assessed at two additional timepoints mid-treatment.