# Pre-specified sample sizes for animal studies; no more 'representative' experiments

Source: https://onco.cc/ideas/idea-tr2-animal-power-mandate/  
OnCo record `idea-tr2-animal-power-mandate` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Underpowered mouse experiments give exaggerated positive results and uninformative negatives, and papers often show one 'representative' result out of several attempts. Funders and journals should require a pre-specified power calculation, the number of independent repeats performed, and reporting of every repeat rather than the best one.

## Summary

Underpowered animal studies produce exaggerated effects when positive and are uninformative when negative. Sample-size justification is required by ARRIVE and by ethics committees in principle but is rarely checked. Funders and journals could require a pre-specified power calculation, the number of independent repeats performed, and reporting of all repeats rather than a representative one.

## Fields

- Kind: Idea
- Last checked: 2026-09-08
- Hypothesis: Enforcement increases the median group size in published in vivo efficacy studies and reduces reported effect sizes towards those seen in adequately powered replications.
- Rationale: Effect-size inflation from small samples is a mathematical certainty under publication bias; the fix is procedural and cheap.
- Proposed test: Adopt at two funders; audit group sizes, repeat reporting and effect sizes in funded publications before and after.
- Maturity: early-clinical
- Actor: policy

## Sources

- ARRIVE guidelines: https://arriveguidelines.org/

## Connected records

- ideas: [Audit animal studies for randomisation and blinding, published by institution](https://onco.cc/ideas/idea-tr2-arrive-audit/)
- technologies: [Patient-derived xenografts](https://onco.cc/technologies/pdx-models/)
- bottlenecks: [Lab models that fail to predict what happens in patients](https://onco.cc/bottlenecks/b-preclinical-models/), [Preclinical results do not reproduce](https://onco.cc/bottlenecks/b-reproducibility/)

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