# 2. Sample size Specify the exact number of experimental units allocated to each group, and the total number in each experiment. Also indicate the total number of animals used. explanation

The sample size relates to the number of experimental units in each group at the start of the study, and is usually represented by *n* (see **item 1 – Study design** for further guidance on identifying and reporting experimental units). This information is crucial to assess the validity of the statistical model and the robustness of the experimental results.

The sample size in each group at the start of the study may be different from the *n* numbers in the analysis (see **item 3 – Inclusion and exclusion criteria**), this information helps readers identify attrition or if there have been exclusions, and in which group they occurred. Reporting the total number of animals used in the study is also useful to identify if any were re-used between experiments.

Report the exact value of* n* per group and the total number in each experiment (including any independent replications). If the experimental unit is not the animal, also report the total number of animals to help readers understand the study design. For example, in a study investigating diet using cages of animals housed in pairs, the number of animals is double the number of experimental units.