A supplier of plastic pellets claims that when the plastic is pressed into wafers of uniform thickness, they will have a mean breaking strength of 165 newtons and a standard deviation of 5 newtons. An engineer at the company randomly selects wafers from 12 batches of pellets.

Question

A supplier of plastic pellets claims that when the plastic is pressed into wafers of uniform thickness, they will have a mean breaking strength of 165 newtons and a standard deviation of 5 newtons. An engineer at the company randomly selects wafers from 12 batches of pellets.
a)Find the probability that the sample mean is less than 162 newtons.
b)Find the probability that the sample mean is between 164 and 166 newtons inclusive.
c)What did you assume in order to do this analysis? How should you check this assumption?

PsyDAG · Answer

Z = (sample mean-mean)/SEm (standard error of the mean)

SEm = SD/√(n-1)

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportions related to the Z scores.