The use of self-administered mindfulness interventions has increased in recent years. The effectiveness of these interventions on regulating stress/emotions, however, is debated. In the present multi-site study (Nsites = x, Nparticipants = x), the investigators aimed to investigate the effectiveness of four single, brief stand-alone mindfulness exercises in a population unfamiliar with mindfulness meditation. The investigators tested these four interventions in comparison to non-mindful active control conditions using an adaptive Bayesian design. The investigators found [evidence for the efficacy of x exercises/no evidence for the efficacy of x exercises] with an estimated mean effect size of [xx/xx]. This means that… or The investigators recommend that… [recommendation will be provided].

Author's note:

The current project was pre-registered on the Open Science Framework (OSF) on March 22nd 2022, which was prior to the start of data collection (https://doi.org/10.17605/OSF.IO/UF4JZ). Data collection started on March 23th 2022 and ended on June 30th 2022. The investigators are retroactively registering the current study upon request of the editor who reviewed their manuscript (to comply with the editorial policies and formatting requirements of the target journal). The following protocol is an abbreviated version of the one the investigators uploaded on the OSF (https://osf.io/us5ae).

The upcoming multi-site project 'Self-administered mindfulness interventions reduce stress in a large, randomized controlled multi-site study' will aim to deliver in-depth insights into how self-administered mindfulness exercises impact stress reduction. It will focus on evaluating the effectiveness of self-administered mindfulness exercises in reducing stress among a large and diverse group of participants. Participants will be randomized to one of the audio-mindfulness exercises or to an active control condition. All exercises and the active control condition instructions will be designed to last 15 minutes. The active control condition will consist of a matched non-mindful listening condition in which mindfulness will not be mentioned. After the end of this intervention, a questionnaire will assess the self-reported stress of individuals with a short-term efficacy span. The study will also investigate the potential role of neuroticism as a moderator, as previous research has negatively associated neuroticism with mindfulness traits (i.e., the ability to be grounded in the present moment; Walsh et al., 2009). Bayesian sequential analysis tools (Schönbrodt et al., 2017) will be used to monitor the progress and to test the efficacy of each self-administered mindfulness exercise (i.e., upon reaching a predefined Bayes Factors). The results of this multi-site project will provide an answer to whether brief, stand-alone mindfulness exercises can demonstrate efficacy in decreasing levels of stress in individuals.

• Inclusion Criteria:
• Adult participants aged 18 years or older at the time of the study.
• Participants who are English native speakers or who self-assessed their English language proficiency at the C1/C2 levels from the Common European Framework of Reference for Languages.
• Exclusion Criteria:
• Participants with a current or past history of mental illnesses (assessed via a pre-screening question).
• Participants who have engaged in meditation within six months prior to data collection.
• Participants who do not meet the English language proficiency requirements specified in the 'Inclusion Criteria' section.

There is no plan for IPD for the current study.

Participants were not aware of the experimental condition in which they were randomly allocated. Data collection was performed blind to the experimental conditions, but data analysis was not performed blind. However, given that all analyses were pre-registered, it is unlikely that the lack of blinding in data analysis introduced bias.