Confidence Interval Methodology with StatKey

For an estimate of the mean, select CI for Single Mean, Median, St.Dev.

Either select one of their data sets, edit data replacing their data with yours, or Upload a data file.
Generate 1000 Samples, and repeat this several times, accumulating at least 5000 samples.
Select the Two-Tail option in the upper left corner of of the dotplot.
The default is to mark 2.5% of samples in the low end and 2.5% in the high end, leaving the middle 95% of the samples.
Here I have found a 95% confidence interval between 98.054 and 98.474.
If you want a 90% confidence interval instead, the click on the middle value , type the percent you want as a decimal, for example, 0.9.
You should notice that the tail percents change as well as the endpoints at the bottom by the x-axis.
Now I have a 90% confidence interval between 98.090 and 98.438.

For an estimate of a proportion, select CI for Single Proportion

Either select one of their data sets, or edit data replacing their data with yours.
Generate 1000 Samples, and repeat this several times, accumulating at least 5000 samples.
Select the Two-Tail option in the upper left corner of of the dotplot.
The default is to mark 2.5% of samples in the low end and 2.5% in the high end, leaving the middle 95% of the samples.
Here I have a 95% confidence interval between 0.420 and 0.620