Confidence Interval
for a Population Mean

Example 1. The Minitab
worksheet Sharks.mtw contains the lengths (in feet) of

44 sharks. Find a 95% t confidence interval for the mean length of the
shark population.

Example 2. The Minitab
worksheet cephalothin.mtw contains the potency measurements for

sixteen doses of the cephalosporin antibiotic cephalothin. Find a
95% confidence interval

for the mean potency in the relevant cephalothin dose population. What assumptions are you making?

What is the correct interpretation of your interval and the correct interpretation of "95%" ?

**Example 3.** Find a 99% confidence interval for the mean cephalothin potency. Compare with the

interval in Example 1.

What Sample Size Do I
Need ?

Example 4. In Example 3 we got a 99% confidence interval for mean cephalothin

potency as (901.50, 914.00), which is an interval with margin of error
6.25. The FDA wants

your next analysis on cephalothin to produce an interval with margin of
error 4.0. at the same

desired confidence of 99%. What sample size (i.e. how many cephalothin doses) do you need

in your next analysis?