Online Mindfulness-Based Cognitive Therapy in Depressed Sample

The Patient Health Questionnaire (PHQ-9): It is a brief self-administered tool for screening and assessing depression severity according to the DSM criteria (Kroenke et al., 2001). Respondents rate the statements on a 4-points scale, yielding a score between 0 and 27.

To increase the reliability of diagnosis, second tool measuring depression will be used: The Center for Epidemiologic Studies Depression Scale (Radloff, 1977). It is a 20-item instrument with each item rated on a four-point scale ranging from 0 ("rarely or none of the time") to 3 ("most or all of the time").

Generalized Anxiety Disorder (GAD-7) scale: it is a brief tool that consists of seven items measuring worry and anxiety symptoms. (Spitzer et al., 2006). Each item is scored on a four-point Likert scale (0-3) with total scores ranging from 0 to 21 with higher scores reflecting greater anxiety severity.

Self-compassion will be measured with a short form of Self-Compassion Scale (SCS-SF; Raes et al., 2011). The tool consists of 12 items (eg. 'I try to see my failings as part of the human condition.') to which person refers on a five-point scale.

The level of resilience will be computed with polish Resilience Measurement Scale (The Resiliency Assessment Scale, SPP-25; Ogińska - Bulik, Juczyński, 2008). The scale consists of 25 statements (e. g. I can easily adapt to new situations) to which respondent relates on a five-point Likert scale.

Five Facet Mindfulness Questionnaire-15 (FFMQ-15): It is a short tool to measure mindfulness as a trait. The items will be measured on a 5-point Likert-type scale from 1 (never or rarely true) to 5 (very often or always true). Example items include: "I do jobs or tasks automatically without being aware of what I'm doing"

The Ruminative Response Scale (RRS): It is a 22-item tool measuring responses to depressed mood that are self-focused, symptom-focused, and focused on the possible causes and consequences of dysphoric mood (Nolen-Hoeksema & Morrow, 1991). Each item is rated on a Likert scale ranging from 1 (almost never) to 4 (almost always).

The Cognitive Fusion Questionnaire (CFQ): It is a 7-item universal tool measuring cognitive fusion and was created as an alternative to questionnaires that were embedded in narrowed-down contexts, such as anxiety disorders or youth (Gillanders et al., 2014).

The Brief Experiential Avoidance Questionnaire (BEAQ): It is a 15-items assessment that was created to measure EA (Gámez et al., 2014). Participants respond to the statements on a 6-point Likert scale (1: "strongly disagree"; 6: "strongly agree").

EMA is a process measure that allows to deepen the understanding of mechanisms of change in the intervention. Our EMA consists of:

• 4 questions about depression
• 3 questions about mindfulness
• 4 questions about ruminations
• 1 affect scale

Working Alliance Inventory tailored to the Internet condition (WAI-TECH-SF): it is a 12-item questionnaire designed to assess the therapeutic alliance with the online program. Participants will response on a seven-point Likert scale, ranging from 1 (never) to 7 (always) (Herrero at al., 2020).

Credibility and Expectancy Questionnaire (CEQ) is a 6-item tool assessing treatment expectancy based on the participant's beliefs. The questionnaire consists of two factors: expectancy (with three questions rated on a 10-point scale, ranging from 1 to 9) and credibility (with one question rated on a 10-point scale and two questions rated on a 1-100% scale) (Devilly, Borkovec, 2000).

The Attitudes towards Psychological Online Interventions Questionnaire (APOI): it is a 16-item tool that was created to assesses respondents' acceptance of Internet interventions along four dimensions (Skepticism and Perception of Risks, Confidence in Effectiveness, Technologization Threat, and Anonymity Benefits). The answers are collected on a five-point Likert-scale (1 = totally agree, 2 = rather agree, 3 = not sure, 4 = rather disagree, 5 = totally disagree)(Schröder et al. 2015) .

A Locus of Control Specific to Well-Being (WB-LOC12): it is a 12-item tool that aims to identify the patients' beliefs concerning the main forces influencing their well-being (Farnier et al., 2021)

The Multidimensional Scale of Perceived Social Support (MSPSS:it is a 12-item tool developed by Zimet and collaborators (1988) as a self-report measure to assess per- ceived social support.