Up to three standard deviations above or below the centerline is the amount of variation that statistical process control allows for

A) Type I errors.
B) about 95.5% variation.
C) natural variation.
D) all types of variation.
E) assignable variation.

The amount of variation allowed for in statistical process control is known as natural variation, which is measured by the spread around the centerline in terms of standard deviations. Typically, the range of acceptable variation is considered to be up to three standard deviations above or below the centerline. This range accounts for about 99.7% of all data points, leaving only about 0.3% outside this range as outliers. Therefore, the correct answer is C, natural variation.