Issue 01 · HDRS · ISO 23698
How many volunteers does an HDRS screening need?
Screenings usually run on 3, 4 or 5 volunteers, more by habit than by calculation. We analysed our own data to quantify what each number provides.
What our data shows
Deviation from the full-study SPF decreases steeply up to 5 volunteers and flattens beyond.
What it means in practice
- 5 volunteers is the efficient size. In our dataset the screening falls within approximately 13 % of the full-study SPF in 9 of 10 cases. Additional volunteers improve precision only marginally.
- 3 volunteers discriminate between prototypes whose SPF differs by approximately 20 % or more. Suitable for ranking candidates; insufficient for a pass/fail decision on a single formula.
- Close to the target (for example SPF 50), a screening result is inconclusive by design. The full study is required there; clearly above or clearly below, the full study rarely changes the conclusion.
The fine print: HDRS and in vivo ISO 24444 agree on average but can differ for specific formulas, mainly low-SPF and mineral products.2,3 We therefore measure that offset on your own portfolio before relying on screening results.
At ISD Research we size each screening to the decision it has to support, ranking prototypes or a go/no-go, and report the uncertainty attached to the result.
Ask how we would size your study
References
- ISD Research data, 55 ISO 23698 studies (2026), mainly emulsions SPF 30–70; bootstrap resampling, 1–20 volunteers.
- Ruvolo E et al. Int J Cosmet Sci 2025;47(S1):53–77. doi:10.1111/ics.13089
- Reble C et al. Int J Cosmet Sci 2025;47(Suppl 1):96–114. doi:10.1111/ics.70007
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Disclaimer. The figures in this article are derived from ISD Research's internal dataset and reflect the formulations tested to date; they are descriptive of that dataset and will be updated as it grows. They are provided for information and do not replace a full ISO 23698 or ISO 24444 study, nor constitute a basis for regulatory SPF claims or labelling decisions. Screening results should always be interpreted together with the product type, the target SPF and the uncertainty reported for each study.