7.15 Finding the critical value t*. What critical value t* from Table D should be used to calculate the margin of error for a confidence interval for the mean of the population in each of the following situations?

1. (a) A 95% confidence interval based on n = 15 observations.

2. (b) A 95% confidence interval from an SRS of 28 observations.

3. (c) A 90% confidence interval from a sample of size 28.

4. (d) These cases illustrate how the size of the margin of error depends upon the confidence level and the sample size. Summarize these relationships.