Can You Average Percentages? (Statistical Rule)
One of the most common mathematical pitfalls is taking a simple average of percentages when the underlying group sizes are unequal.
For instance, if Group A has 10 people with an 80% pass rate, and Group B has 1,000 people with a 90% pass rate, averaging (80% + 90%) / 2 = 85% is statistically inaccurate because Group B contains 100 times more people. You must use a weighted average.
Weighted Average of Percentages Formula
Weighted Average = (P₁ × W₁ + P₂ × W₂ + ... + Pₙ × Wₙ) / (W₁ + W₂ + ... + Wₙ)