The Excel CONFIDENCE.NORM Function

Related Function:
CONFIDENCE.T

Confidence Interval
In statistics, the confidence interval is the range that a population parameter is likely to fall into, for a given probability.

Eg. For given population and a probability of 95%, the confidence interval is the range, in which a population parameter is 95% likely to fall.

Note that the accuracy of the confidence interval relies on the population having a normal distribution.

The Confidence Interval is discussed further on the Wikipedia Confidence Interval Page


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.

The format of the function is :

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.

  A
1 =CONFIDENCE.NORM( 0.05, 0.07, 100 )
2  

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


Further information and examples of the Excel Confidence.Norm function can be found on the Microsoft Office website.


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 :

Common Errors
#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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