Let's say we have the following statistics from a data set and we want to find the 95% confidence interval for mux, the underlying population's true average value.
n = 36
x-bar = 25.35
sx = 6.78
The degrees of freedom will be n-1 = 35, but 35 is not one of the choices on Table A-3, so we have to go with row 34 instead. The Confidence Level Multiplier for 95%, or CLM95%, is 2.032. If you have a TI-30xIIs, key in the following numbers into your calculator to get the high threshold.
25.35+2.032*6.78/[2nd][x^2]36[enter]
The answer should be 27.64616, which we can round to 27.65.
All we have to do to get the low threshold is to change the + to a -. Key in the following.
[up][right][right][right][right][right]-[enter]
After you key this in, the equation line should look like
25.35-2.032*6.78/sqrt(36
and the answer is 23.05384, which rounds to 23.05.
This says we are 95% confident using this data set that the true average of the underlying population is between 23.05 and 27.65.
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