Sample Size for Comparing Two Groups

Most studies come down to comparing two groups. This level gives you the ingredients and the hands-on demos to size two-means and two-proportions comparisons, then adjust for the realities of unequal groups and dropout.

Who this level is for

Researchers and clinicians designing comparative studies or trials, and anyone who reviews them. Recommended after Levels 1-2.

What you'll learn

• The ingredients a two-means calculation needs: group means or their difference, standard deviation, alpha and power
• A hands-on two-means demo (for example, an HbA1c difference between two treatments)
• The ingredients for a two-proportions calculation, and difference versus ratio (risk difference vs relative risk)
• A hands-on two-proportions demo (for example, 25% vs 35% mortality)
• How unequal allocation and loss to follow-up change the number you need

You finish with: The sample size for your study's main two-group comparison.