The Compassion Scale assesses compassion for others in similar dimensions to self-compassion. Responses are given on a 5-point scale ranging from 1 (almost never) to 5 (almost always). Pommier's (2010) results suggest that this scale presents good internal consistency and adequate convergent validity. A Spanish adaptation of Pommier's scale was developed for Brito-Pons (2014). The English version was translated by the researcher and refined with a team of psychologists and mindfulness instructors following a committee approach (Graham et al. 2003); finally, it was back-translated by native English speakers. Higher scores indicate greater compassion.

The FSCRS-SF contains 14 items to assesses two forms of self-criticism: inadequate self and hated self, and the ability to self-reassure.

Participants respond to a selection of statements, asking about how one thinks and reacts in the face of failures or setbacks, on a 5-point Likert scale ranging from 0 (not like me at all) to 4 (extremely like me). Higher scores indicate a greater sense of inadequacy, self-hate, or self-reassurance. The FSCRS-SF subscale scores had adequate test-retest reliability and satisfactory convergent validity estimates with theoretically related constructs.

The FFMQ-15 is a short version of the FFMQ-39. This version includes 15 items to assess dispositional mindfulness rated on a Likert scale ranging from 1 (never or very rarely true) to 5 (very often or always true).

Five facets of mindfulness are assessed: Observe, refers to the subject's capacity to pay attention to internal and external experiences such as sensations, thoughts, or emotions; Describe, measures the ability to describe events and personal responses in words; Acting with awareness, includes focusing on the activity being carried out, as opposed to behaving automatically; Non-judging of inner experience, refers to the ability to take a non-evaluative stance toward thoughts and feelings; and Non-reactivity to inner experience, allowing thoughts and feelings to come and go, without getting caught up in them or carried away by them. The five scales show good internal consistency. Higher scores represent higher compassion.

The RQ uses a 7-point Likert-scale that assesses and matches participants with one of four attachment styles: (i) secure, (ii) pre-occupied, (iii) dismissive and (iv) fearful. A mathematical calculation permits a categorical assessment of attachment style (i.e. secure or insecure) (Griffin & Bartholomew 1994), and qualitative self-descriptor criteria can be used for confirmatory purposes. Studies have demonstrated that the reliability of the self-descriptor criteria is high (Leak and Parsons 2001; Yarnoz-Yaben and Comino 2011). The RQ also offers the possibility of measuring two key dimensions underlying attachment in adults (Griffin and Bartholomew 1994a)-namely, anxiety, which relates more to the self, and avoidance, which relates more to others (Griffin and Bartholomew 1994b). The anxiety dimension is calculated using the sum of the four attachment style ratings. High scores in this dimension reflect high anxiety towards social relationships.

GHQ-12 is one of the most widely used screening tests to assess mental health of a population, and is designed as a structured, brief and self-administered questionnaire.

Every one of its 12 items regarding recent symptoms, feelings or behaviors is answered on a four-category Likert scale. Categories 1 and 2 are given value 0, and categories 3 and 4 are given value 1. Values from 12 items are added together to get an overall score. A probable psychiatric case is considered when the score is equal to or greater than 3.

The NAS-7 is scored on a 6-point scale ranging from 1 = disagree strongly to 6 = agree strongly and designed to measure nonattachment (e.g., I can let go of regrets and feelings of dissatisfaction about the past when pleasant experiences end, I am fine moving on to what comes next).

The NAS-7 is unifactorial and shows excellent internal consistency (α = .82).

Higher scores represent higher nonattachment values.

The I-PANAS-SF -a reduced version of the PANAS- is composed of 10 items with 5-item Positive Affect (PA) Negative Affect (NA) subscales.

In a series of validation studies (N = 1,789), the cross-sample stability, internal reliability, temporal stability, cross-cultural factorial invariance, and convergent and criterion-related validities of the I-PANAS-SF were examined and found to be psychometrically acceptable.

Higher scores of PA subscale represent higher positive affect. Higher scores of NA subscale represent higher negative affect.

The PIL test is a 10-item self-report attitudinal scale designed to measure the extent to which a respondent perceives a general sense of meaning and purpose in life or respectively suffers from an "existential vacuum". The Spanish validation has shown to be a reliable measure to assess the meaning of life. Higher scores represent a higher sense of meaning and purpose in life.

The Compassion Practice Quality Questionnaire has been developed for this study to assess difficulties related to the practice of compassion meditation. This self-reported questionnaire includes 10 items that participant scores on a scale ranging from 0 to 100 indicating the percentage of time that his/her experience reflects each statement. Higher scores represent higher difficulties related to the compassion meditation practice.

This instrument was adapted from the System Usability Scale (SUS) in order to assess the usability of a service or product and the acceptance of technology by the people who use it. The SUS has been shown to be a valuable and robust tool for assessing the quality of a wide range of user interfaces, as it is easy to use and understand. This scale includes 10 statements rated on a five-point scale measuring agreement with the statement (0 = strongly disagree; 4 = strongly agree). The final score is obtained by adding the scores on each item and multiplying the result by 2.5. Scores range from 0 to 100, where higher scores indicate better usability.

Qualitative Interview has been specifically developed to assess participant opinions about the internet intervention. This interview included 14 questions: seven of them regarding usefulness of the intervention, components, modules, information provided, and multimedia elements (e.g., images, audios, videos, pdf) rated on a scale from 1 to 5 (1 = very little; 2 = little; 3 = something; 4 = a lot; and 5 = very much) and two dichotomous questions ("yes" or "no") regarding whether they would like having at their disposal the program for more time after the treatment has finished, as well as whether they would like to have longer access. Additionally, options to extend the participants' qualitative responses were available. Finally, two open questions are included to assess advers or unwanted effects and facilitators and barriers towards the intervention received.