Thread: Confidence Levels

A set of metal bars are known to have a mean diameter of 13.03mm with standard deviation of 0.63mm. If it can be assumed that diameters are normally distributed:
i. find the 90% and 95% confidence levels for the diameter of an individual bar.

is this 13.03 +- 1.96(0.63) and 13.03 +- 1.64(0.63) ?

ii. What proportion of the bars will have a diameter greater than 13.75mm?

Originally Posted by tone999

A set of metal bars are known to have a mean diameter of 13.03mm with standard deviation of 0.63mm. If it can be assumed that diameters are normally distributed:
i. find the 90% and 95% confidence levels for the diameter of an individual bar.

is this 13.03 +- 1.96(0.63) and 13.03 +- 1.64(0.63) ?

They should be the other way around is the interval and the interval

ii. What proportion of the bars will have a diameter greater than 13.75mm?

The z score for is:



which you now look up in the table of the cumulative standard normal distribution and the required probability is .

CB