Supplementary Exercise 7.1 of IPS7e
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Values of t* for confidence intervals for a mean. In all cases, the
model assumptions should be of a sample of i.i.d. observations from a
Normal distribution with unknown mean and variance.

(a) 90% CI based on a sample size of n=12. 
The degrees of freedom are: df = 12-1 = 11. 
Table C of PSLS gives t*=1.796.

(b) 95% CI based on a sample size of n=30. 
The degrees of freedom are: df = 30-1 = 29. 
Table C of PSLS gives t*=2.045.

(c) 80% CI based on a sample size of n=18. 
The degrees of freedom are: df = 18-1 = 17. 
Table C of PSLS gives t*=1.333.

The simplest way to get these values in Minitab is from the Probability
Distribution Plot menu and its submenu View Probability. After choosing
the relevant t-distribution, one can insert the error level (100% minus
confidence level) and choose Both Tails.