Single-Session Intervention Targeting Self-Injurious Behavior in Adolescents

Inclusion Criteria

• Must be 13 - 16 years old at the time of the baseline survey
• Must report comfort reading and writing in English
• Must endorse no learning disability, visual impairment, or other difficulty that makes it difficult to answer questions on a computer
• Must have access to a laptop or smartphone with internet access
• Must endorse purposefully hurting self without wanting to die within the past month
• Must endorse either "I do not like myself" or "I hate myself" on CDI-II item 7 in a screener survey

Exclusion Criteria

Type 1 exclusions are exclusions that can be automated and do not require consensus among investigators. A research team member who will not be involved with conducting the primary study analyses will use type 1 exclusions to determine when participant recruitment is complete. Recruitment for the present study will end once 500 participants have been randomized who pass all type 1 exclusions tests (i.e. are still eligible for inclusion after excluding for type 1 criteria; see our section on sample size for more details).

Type 1 exclusions include:

• Not meeting inclusion criteria
• Participants who exit the study prior to condition randomization for our listed analyses (note: the present study will use an intention-to-treat approach, where every participant who was randomized to a study condition-and who is not excluded for one of the specific reasons outlined in this section-will be included our our analyses)

Type 2 Exclusions

Type 2 exclusions are exclusions that cannot be automated and require consensus among investigators. Participants excluded for type 2 reasons will be subtracted from the N = 500 total individuals who were randomized and met criteria for inclusion following type 1 exclusions.

Here, we will exclude the following participants based on failure to meet the following quality check criteria:

Participants who respond with either copy/pasted responses from text earlier in the intervention (e.g. copy and pasting only text from a previous testimonial slide) to any of free response questions Participants demonstrating an obvious lack of English fluency in open response questions; these participants will be identified by consensus from all study team members Participants responding with random text in open response questions; these participants will be identified by consensus from all study team members Duplicate responses from the same individual in baseline or follow-up surveys (i.e. more than one response with an identical IP address). Where duplicate responses for the same individual are present in the same survey (e.g. more than 1 response for a single individual at follow-up), we will exclude the response that is less complete, retaining the more complete of the two responses. If both responses are 100% complete, we will retain the first of the two responses for that survey. Notably, if an individual completes the baseline survey more than once-and happens to be randomized to both conditions-we will exclude this individual's responses from our analysis altogether.

We will also exclude for primary analyses (but may run sensitivity analyses including them) any participants who provide responses of 3 words or fewer to writing prompts that ask for at least 2 sentences or more.

These exclusions are based on previous single session intervention research conducted online

(for one example, see Schleider & Weisz, 2018: https://doi.org/10.1080/15374416.2017.1405353)