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Are We Losing the Most Relevant Cases First? Selective Dropout in Two Longitudinal Studies of Adolescent Pornography Use


In longitudinal studies of pornography use, selective loss of participants who may be more vulnerable to the effects of pornography than their peers is a serious concern. To explore the potential for such selective dropout, we used data from two independent large-scale panel studies of adolescents’ pornography use. Of the three types of attrition—early attrition, later attrition, and gaps in participation—only the first was substantially higher among more vulnerable adolescents, compared with other participants. Panel type (online vs. classroom-based) moderated only the association between vulnerability and participation gaps, which was significant in the classroom-based but not the online panel. Overall, this study’s findings point to the importance of delaying selective dropout by developing a comprehensive plan of action, for which we offer some guidelines.

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  1. According to the national guidelines for ethical research with minors (Kolesarić & Ajduković, 2003), if participants are aged 14 years or older, their parents need only be informed about the study; informed consent should be obtained from participants.

  2. The final study wave, in which the distinction was logically impossible, served to differentiate between attrition and participation gaps in the previous one. In addition, observations for students who attended a 3-year vocational program (the majority of our participants were enrolled in a 4-year secondary-school program) were right-censored after T3 in the Zagreb panel and T4 in the Rijeka panel.

  3. Faced with the unexpectedly high dropout, the authors decided to end the study (personal communication with L. Kuyper).

  4. The PROBIOPS study created a Facebook group to maintain contact with participants, mainly by periodically posting infographics with (non-central) study findings.


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This work has been fully funded by Croatian Science Foundation (Grant Number 9221 awarded to the first author).

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Correspondence to Goran Koletić.

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The authors confirm that all relevant ethical safeguards have been met in relation to subject protection including informed consent and approval by the Committee on Ethics Issues in Science and Research of the Faculty of Humanities and Social Sciences, University of Zagreb. The Ethics Issues Confirmation was issued on 2 June 2014.

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Štulhofer, A., Matković, T., Kohut, T. et al. Are We Losing the Most Relevant Cases First? Selective Dropout in Two Longitudinal Studies of Adolescent Pornography Use. Arch Sex Behav 50, 2215–2226 (2021).

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  • Adolescents
  • Pornography use
  • Panel attrition
  • Selective dropout