Marlene Kritz

Marlene Kritz

Marlene Kritz is a physical activity researcher at the University for Continuing Education Krems in Austria, and the first author of the largest direct comparison of group-based and individual exercise interventions yet published. Her 2026 meta-analysis in Nature Human Behaviour pooled 71 studies and 523 effect sizes, and on physical activity outcomes across 22,042 people it found no significant advantage for the group: g = 0.086, 95 per cent CI −0.061 to 0.233, p = 0.249. The interval crosses zero. That single result is why the article she appears in was retitled and rebuilt.

Who they are

Her paper sits alongside the work of Nikos Ntoumanis and Cecilie Thøgersen-Ntoumani on self-determination theory in physical activity, both of whom are co-authors on it, and it complements that line of work rather than repeating it. Their meta-analyses ask which psychological needs an intervention can move. Hers asks whether the delivery format matters at all.

Her affiliation is corrected here. Article 10's people block originally gave Curtin University, which is where several of her co-authors sit. Checked against the Nature Human Behaviour record on 11 September 2026, she is at the University for Continuing Education Krems. The given name "Marlene" was originally inferred from the author group rather than read off the paper, and was confirmed in the same pass.

Key papers

Kritz, M., Riddell, H., Olsen, D., Harden, S. M., Burke, S. M., Ntoumanis, N., & Thøgersen-Ntoumani, C. (2026). "Individual versus group-based interventions: a systematic review and meta-analysis of physical activity, functional, psychosocial and health outcomes", Nature Human Behaviour, 10(6), 1109 to 1121. First author. Searches completed 19 March 2024. 71 studies, 523 effect sizes. On physical activity outcomes, n = 22,042, g = 0.086, 95 per cent CI −0.061 to 0.233, p = 0.249, a small and non-significant advantage for group-based conditions.

Key ideas

This is the most useful null in the vault's exercise cluster. It does not say that exercising with other people fails. It says that "delivered to more than one person at a time" is too coarse a variable to predict anything, which is a finding about measurement as much as about behaviour.

The design forces walking groups, supervised classes, group counselling and team-based programmes into a single category. Anybody who has been in a spin class and in a Sunday league team knows they are not the same object, and the meta-analysis has no way to say so.

Both sides, then the line. On one side, Shauna Burke and colleagues reported in 2006 that groups run deliberately on group-dynamics principles outperformed individual approaches, and a 2025 randomised trial in youth found a group advantage of more than 10 MET-hours per week. On the other, some trials at twelve months find home-based programmes beating group conditions on participation. We land with the 2026 meta-analysis, because it is larger, newer and directly on the comparison: group versus solo as a category does not hold, and the specific form of the group is where the question moved.

The honest use of a null is to ask what it averaged away, not to argue with it.

Concepts on this site

Exercise Adherence, the outcome the comparison was testing

Group Identification, the narrower construct the null makes necessary

Self-Determination Theory, the adjacent motivational account her co-authors work in

Sources

  1. Kritz, M., et al. (2026). Nature Human Behaviour, 10(6), 1109 to 1121.
  2. Burke, S. M., Carron, A. V., Eys, M. A., Ntoumanis, N., & Estabrooks, P. A. (2006). Sport and Exercise Psychology Review, 2(1), 19 to 35. The other side of the question, reported at one remove and without numbers.
  3. Vault verification record, 2026-09-11. Given name confirmed and affiliation corrected from Curtin University to the University for Continuing Education Krems, Austria, in article.json and the matching entities file.