How many rats should you take? - briefly
Three to five rats generally provide adequate statistical power for most experimental designs while adhering to ethical standards. Adjust the count only if the study’s effect size, variability, or specific regulatory requirements dictate otherwise.
How many rats should you take? - in detail
Determining the appropriate number of rats for an experiment depends on statistical power, effect size, variability, and ethical constraints.
First, define the primary endpoint and calculate the expected difference between control and treatment groups. Use published data or pilot studies to estimate the standard deviation. Apply a power analysis (commonly 80 % or 90 %) with a significance level of 0.05 to obtain the minimum sample size per group.
Second, consider the experimental design.
- For a simple two‑group comparison, the total number equals twice the per‑group requirement.
- For factorial or repeated‑measure designs, adjust the calculation to reflect within‑subject correlations and additional factors.
Third, incorporate attrition rates. If historical loss is 10 %, increase the calculated sample size accordingly to maintain statistical power.
Fourth, respect the 3Rs (Replacement, Reduction, Refinement). The calculated minimum should not be exceeded without justification. Document the power analysis, assumptions, and ethical review in the study protocol.
Finally, verify compliance with institutional animal care guidelines, which may impose upper limits on group sizes or total animal numbers. Adjust the plan if any regulatory restriction conflicts with the statistical recommendation.
By systematically applying these steps, researchers arrive at a defensible, ethically sound number of rats for their specific investigation.