Theobald's Rule

Theobald's Rule is a principle used in trial design, particularly for determining how many participants are needed to reliably detect a real effect. It suggests that the required sample size increases proportionally with the number of different treatment groups being tested. In other words, if a study compares more groups, you need more participants to keep the results statistically sound. This helps researchers plan studies that are efficient but still capable of accurately identifying meaningful differences or effects among treatments.