Q
I thought this would be a good question for the stats experts at the Cove. 
We've tested 20 samples of slow turning motors that we buy for fourteen days, and we've come up with an average # of degrees per day of rotation for each motor.
What I would like to do is present to Engineering a 99% confidence interval range of where I feel these motors run, based on these 20 samples, (see attached data).
When I perform the calculations using a t value of 2.861, (.995 @ 19 DOF), I get a 99% confidence range from 25.70 to 25.75 deg/day.
If I assume the stdev of my sample is equal to the population stdev, however, and I use a Z table I get a 99% confidence range from 25.62 to 25.83 deg /day.
I feel much more comfortable with the Z calculated range than I do the t calculation. Can anyone tell me why? and am I justified to report the Z calculated range?
Thanks in advance.........
We've tested 20 samples of slow turning motors that we buy for fourteen days, and we've come up with an average # of degrees per day of rotation for each motor.
What I would like to do is present to Engineering a 99% confidence interval range of where I feel these motors run, based on these 20 samples, (see attached data).
When I perform the calculations using a t value of 2.861, (.995 @ 19 DOF), I get a 99% confidence range from 25.70 to 25.75 deg/day.
If I assume the stdev of my sample is equal to the population stdev, however, and I use a Z table I get a 99% confidence range from 25.62 to 25.83 deg /day.
I feel much more comfortable with the Z calculated range than I do the t calculation. Can anyone tell me why? and am I justified to report the Z calculated range?
Thanks in advance.........
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