| Question : Why don't statisticians calculate 100% confidence intervals? Answer : Good question. In short, to be 100% sure of something, you'd need to consider any possible value, so the confidence interval would be the entire range of values. For example, the 100% Confidence Interval for a proportion is 0 to 100. That is, we can be 100% sure the real value we're measuring is between 0 and 100. Not very helpful is it? It doesn't tell you anything you don't already know. Even a 99.9% confidence interval, which is close to 100% and is often used with large sample-sizes would narrow the range of likely values. See the confidence level tutorial to see how the confidence level affects the width of the confidence interval. | How helpful was this answer?
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