Microsoft Excel, a spreadsheet computer program, offers a formula called "CONFIDENCE" to calculate the confidence interval. The interval represents the range from the survey value in which the true proportion is likely to exist. For example, if you found that 72 percent of people prefer Brand A over Brand B with a confidence interval of 3 percent and a 95 percent confidence level, you could say that there is a 95 percent probability that the true proportion lies between 69 and 75 percent.

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Enter the alpha value in cell A1. The alpha value equals 1 minus the confidence level. For example, if your confidence level equals 96 percent, you would enter 0.04 in cell A1.

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Enter the standard deviation in cell A2. For example, if your standard deviation equals 1.1, enter "1.1" in cell A2.

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Enter the sample size in cell A3. For example, if your sample size equals 3,000, enter "3,000" in cell A3.

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Enter "CONFIDENCE(A1,A2,A3)" in cell A4 and Excel will display the confidence interval. In this example, Excel will display "0.041245769" meaning your confidence interval equals plus or minus 4.12 percent.