# WorksheetFunction.Confidence Method

 Excel Developer Reference

Returns a value that you can use to construct a confidence interval for a population mean.

Syntax

expression.Confidence(Arg1, Arg2, Arg3)

expression A variable that represents a WorksheetFunction object.

Parameters

Name Required/Optional Data Type Description
Arg1 Required Double The significance level used to compute the confidence level. The confidence level equals 100*(1 - alpha)%, or in other words, an alpha of 0.05 indicates a 95 percent confidence level.
Arg2 Required Double The population standard deviation for the data range and is assumed to be known.
Arg3 Required Double The sample size.

Return Value
Double

Remarks

The confidence interval is a range of values. Your sample mean, x, is at the center of this range and the range is x ± Confidence. For example, if x is the sample mean of delivery times for products ordered through the mail, x ± Confidence is a range of population means. For any population mean, μ0, in this range, the probability of obtaining a sample mean further from μ0 than x is greater than alpha; for any population mean, μ0, not in this range, the probability of obtaining a sample mean further from μ0 than x is less than alpha. In other words, assume that we use x, standard_dev, and size to construct a two-tailed test at significance level alpha of the hypothesis that the population mean is μ0. Then we will not reject that hypothesis if μ0 is in the confidence interval and will reject that hypothesis if μ0 is not in the confidence interval. The confidence interval does not allow us to infer that there is probability 1 – alpha that our next package will take a delivery time that is in the confidence interval.

• If any argument is nonnumeric, Confidence generates an error.

• If alpha ≤ 0 or alpha ≥ 1, Confidence generates an error.

• If standard_dev ≤ 0, Confidence generates an error.

• If size is not an integer, it is truncated.

• If size < 1, Confidence generates an error.

• If we assume alpha equals 0.05, we need to calculate the area under the standard normal curve that equals (1 - alpha), or 95 percent. This value is ± 1.96. The confidence interval is therefore: