How to Find a Confidence Interval for the True Mean Using Excel 2007

1. Enter the alpha value in cell A1. The alpha value represents '1,' minus the probability the true mean lies within the confidence interval. For example, if you want a 95 percent probability that the true mean lies within the confidence interval, your alpha value equals 0.05.
2. Enter the standard deviation of the data in cell A2. The wider the distribution of data, the greater the standard deviation.
3. Enter the sample size in cell A3. The larger the sample size, the smaller the confidence interval will be.
4. Enter '=CONFIDENCE(A1,A2,A3)' into cell A4. Excel will display the confidence interval. For example, if you have an alpha value of 0.05, a standard deviation of 3 and a sample size of 160, the confidence interval would be 0.464846274.