The 68–95–99.7 Rule of the Normal Distribution
Where class scores fall relative to the mean on a bell curve
−3σ
−2σ
−1σ
μ
+1σ
+2σ
+3σ
±1σ ≈ 68%
±2σ ≈ 95%
±3σ ≈ 99.7%