The Normal Curve
The normal curve is one of the most
common patterns of variation.
The normal curve is symmetrical
around the mean value.
Most of the data points are close to the
mean but the curve tails off in both directions getting closer and
closer to zero.
Standard SPC techniques are based on the normal
curve and its probabilities.
The “68-95-99.7 Rule” applies to all
99.7% of process data fall within ±3 standard
deviations of the mean.
95% of process data fall within ±2
standard deviations of the mean.
68% of process data fall within
±1 standard deviation of the mean.