Statistical Process Control (SPC) Resource Center

The Normal Curve

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 normal curves

  • 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.

The Normal Curve