Suppose a purchasing manager wants to determine the average cost of machined metal parts in the US. How many machined metal parts would they have to sample? The manager wants to be 90% confident with a margin of error of 6.74. Suppose that the costs of the metal parts ranges from 93 to 220. When entering the values into JMP, round the planning value for the standard deviation to four values after the decimal. Round final answer as you normally would.

You can use the margin of error with the z value and standard deviation to get the result.

The number of metal machine parts would be 60.

The number of machined metal parts that have to be sampled.

Range = Max. value - min. value = 220 - 93 = 127

The standard deviation is approximately the quarter of the range of the distribution.

Thus,

For 90% confidence, the z value is 1.645 (from z tables)

Thus, from the formula for size of sample, we have:

The number of metal machine parts would be 60.

Learn more about sample size here: