Basic Description

The Excel CONFIDENCE.NORM function uses a Normal Distribution to calculate a confidence value that can be used to construct the confidence interval for a population mean, for a supplied probablity and sample size. It is assumed that the standard deviation of the population is known. The Confidence.Norm function is new to Excel 2010. However, this is simply a renamed version of the Confidence function, which is available in earlier versions of Excel.

Syntax: CONFIDENCE.NORM( alpha, standard_dev, size )

where the arguments are listed in the table below :

alpha The significance level (=1-confidence level)
– eg. a significance level of 0.05 equates to a confidence level of 95%
standard_dev The standard deviation of the population
size The population sample size


To calculate the confidence interval for a population mean, the returned CONFIDENCE value must then be added to, and subtracted from, the sample mean. ie. for the sample mean x :

Confidence Interval = x ± CONFIDENCE


Confidence.Norm Function Example

In the spreadsheet below, the Excel Confidence.Norm Function is used to calculate the confidence interval, with a significance of 0.05 (ie. a confidence level of 95%), for the mean of a sample of heights of 100 men. The sample mean is 1.8 meters and the standard deviation is 0.07 meters.

1 =CONFIDENCE.NORM( 0.05, 0.07, 100 )


The above function returns a confidence value of 0.013719748

Therefore the confidence interval is 1.8 ± 0.013719748, which is equal to :

1.786280252 to 1.813719748


Confidence.Norm Function Common Errors

If you get an error from your Excel Confidence.Norm function this is likely to be one of the following :

#NUM! Occurs if either the supplied alpha is ≤ 0 or ≥ 1 or the supplied standard_dev is ≤ 0 or the supplied size argument is < 1
#VALUE! Occurs if any of the supplied arguments are non-numeric

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