Using a TI-84 Plus to Find Confidence Intervals

The Zinterval command constructs confidence intervals when the population standard deviation σ is known. This command is accessed by pressing STAT and highlighting the TESTS menu.

calculator screen showing zinterval.PNG

If the summary statistics are given the Stats option should be selected for the input option.

calculator screen showing input for zinterval.PNG

Example Using TI-84

A machine that fills cereal boxes is supposed to put 20 ounces of cereal in each box. A simple random sample of 6 boxes is found to contain a sample mean of 20.25 ounces of cereal. It is known from past experience that the fill weights are normally distributed with a standard deviation of 0.2 ounce. Construct a 90% confidence interval for the mean fill weight.

Solution

We press STAT and highlight the TESTS menu and select Zinterval.

Select Stats as the input option and enter 0.2  σ field, 20.25 for the LaTeX: \bar{x}ˉx field, 6 for the n field, and 0.9 for the C-level field. 

Select Calculate.

calsulator screens for this example using zinterval.PNG

The confidence interval is (20.12,20.38).

We are 90% confident that the mean weight LaTeX: \mu μ is between 20.12 and 20.38.

Here is another example showing the actual calculator, in case it is helpful for you to work through it along with the presenter.