1. A pharmaceutical company producing vitamin capsules desires a proportion of calcium content between 40 and 55 ppm. A random sample of 20 capsules chosen from the output yields a sample mean calcium content of 44 ppm with a standard deviation of 3 ppm. Find the natural tolerance limits of the process. If the process is in control at the present values of its parameters, what proportion of the output will be nonconforming, assuming a normal distribution of the characteristic?
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If the process is in control at the present values of its parameters and the calcium content follows a normal distribution, approximately 0.06% of the output will be non-conforming.
Given:
A pharmaceutical company produces vitamin capsules.
They desire a proportion of calcium content between 40 and 55 ppm.
A random sample of 20 capsules chosen from the output yields a sample mean calcium content of 44 ppm with a standard deviation of 3 ppm.
To find:
Find the natural tolerance limits of the process. If the process is in control at the present values of its parameters, what proportion of the output will be nonconforming, assuming a normal distribution of the characteristic?
Solution:
Formula used:
Upper natural tolerance limit = sample mean + 3 x SD
Lower natural tolerance limit = sample mean - 3 x SD
Z - score z = (x - μ) / σ
From the data,
Sample mean, x = 44 ppm
Standard deviation, SD = 3 ppm
Using the above formulas
The natural tolerance limits of the process can be calculated as follows:
Upper natural tolerance limit = 44 + (3 x 3) = 53
Lower natural tolerance limit = 44 - (3 x 3) = 35
∴ The natural tolerance limits of the process are 35 ppm and 53 ppm.
To find the proportion of the output calculate the z-scores corresponding to the natural tolerance limits
Using the formula z = (x - μ) / σ
For the upper natural tolerance limit:
z = (53 - 44) / (3 / √20) = 3.41
For the lower natural tolerance limit:
z = (35 - 44) / (3 / √20) = - 3.41
Using a standard normal distribution table or calculator, we can find the probabilities beyond these z-scores:
P(Z > 3.40) = 0.0003 (approximately)
P(Z < -3.40) = 0.0003 (approximately)
Therefore, the proportion of nonconforming output is:
P(nonconforming) = P(Z > 3.40) + P(Z < -3.40) = 0.0003 + 0.0003 = 0.0006
Therefore,
If the process is in control at the present values of its parameters and the calcium content follows a normal distribution, approximately 0.06% of the output will be non-conforming.
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