If your sample size is large (n is greater than or equal to 30), use z-value.
If you have a small sample (n < 30), then use t-value.
CI95 = mean + or - 1.96(sd divided by √n)
...where + or - 1.96 represents the 95% confidence interval using a z-table, sd = standard deviation, √ = square root, and n = sample size.
Plug the values from the problem into the formula to find your confidence interval.
I hope this will help get you started.
A random sample of size 49 is taken from a large population, measuring the time it takes to complete a driver’s license examination. The sample mean was found to be 47 minutes, and the sample standard deviation 5.89 minutes. Construct a 95% confidence interval around the sample mean. Be sure to show whether you use the t- or z- value, and what value you use. Even though you are normally “allowed” to use an approximate value of “t” or “z” for this percent confidence, I want you to use the exact value appropriate for these circumstances. Interpret the confidence interval in a single sentence. In another sentence, state why you used either the “t” or the “z”.
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