How to Do the Calculations for a Confidence Interval Using Web Stat
1. Go to http://www.stat.sc.edu/webstat/
it will take a long time (up to a couple of minutes) to load if you are on a dial up connection. The last thing to appear will be the orange "Click here to fire it up!" button.
2. Click on the orange "Click here to fire it up!" button and a gray spreadsheet will appear.
3. Type the data in the "var 1" column like above.
4. After all the data has been entered, but your mouse on "Stat" then slide it down to "T statistic" and then over to "One sample."
5. Click on the small down arrow as indicated by the green arrow in the diagram below and select var 1.
6. Press the "Next" button and the following dialog box will appear. Make sure the "Confidence Interval" radio button is selected (has a black dot in it). Type the confidence level in the "Level:" box.
7. Press the "Calculate" button. The solution will appear at the bottom of the page:
In the above example, the output reads:
95% Confidence interval results:
Variable Estimate Std. Err. DF L. Lim U. Lim
var1 26.666666 1.6261748 5 22.486452 30.846882
This tells us that for this example our sample mean is 26.6667, our standard error is 1.6262, our degrees of freedom are 5, our lower limit is 22.4864 and our upper limit is 30.8468. Of course, you have different data so you will get a different answer. See pages 178 to 184 for information about confidence intervals.
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