Question 1Multiple choiceQuality
A machining process produces shafts whose diameters are normally distributed with a mean of 50.20 mm and a standard deviation of 0.40 mm. The specification limits for the diameter are 49.00 mm to 51.00 mm, and the process is in statistical control. What is the process capability index ?
- A.
0.33
- B.
0.67
- C.
0.83
- D.
1.00
Worked solution
measures how much room the process has between its mean and the nearer specification limit, in units of three standard deviations. It is the smaller of the two one-sided indices, because the side closer to the mean is the one that produces nonconforming parts:
Upper side:
Lower side:
The upper side governs because the mean sits above the centre of the tolerance:
A value below 1 means the process is not capable: the upper limit is only standard deviations above the mean, so about 2.3% of shafts exceed 51.00 mm.
≈ 0.67 (option B)
The most tempting wrong answer is C (0.83), which is . measures the spread of the process relative to the tolerance width only; it ignores where the mean is and would equal only if the process were centred at 50.00 mm. Option D (1.00) takes the lower-side index, but is governed by the nearer limit. Option A (0.33) divides the one-sided distance by instead of .