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The formula used to compute a confidence interval for the mean of a normal population when n is small is the following.

http://i.imgur.com/vluLbC3.gif

What is the appropriate t critical value for each of the following confidence levels and sample sizes? (Round the answers to two decimal places.)

(a) 90% confidence, n = 17

(b) 90% confidence, n = 13

(c) 99% confidence, n = 24

(d) 90% confidence, n = 25

(e) 95% confidence, n = 13

thank you so much in advance!

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    $\begingroup$ Where exactly do you need help with these questions? Please see our help center for guidance on how to ask good, answerable questions. $\endgroup$
    – whuber ♦
    Dec 12, 2013 at 7:23
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    $\begingroup$ Please read the tag wiki info for the self study tag. It asks rather more of you than just posting the question and hoping for answers. $\endgroup$
    – Glen_b
    Dec 12, 2013 at 8:56

1 Answer 1

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Critical t-values are calculated as the alpha/2 quantile of the t-distribution with n-1 degrees of freedom, where alpha denotes the significance level (i.e. 1 minus the significance level gives you the % confidence interval). The t-distribution loses one degree of freedom as you estimate the mean from your sample (one restriction).

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