Basic Description

The Excel CONFIDENCE.NORM function uses a Student’s T-Distribution to calculate a confidence value that can be used to construct the confidence interval for a population mean, for a supplied probablity and supplied sample size. It is assumed that the standard deviation of the population is known. The Confidence.T function is new to Excel 2010, so is not available in earlier versions of Excel. However, earlier versions of Excel do have the Confidence Function, which uses the Normal Distribution to calculate the confidence value.

Syntax: CONFIDENCE.T( 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.T Function Example

In the spreadsheet below, the Excel Confidence.T 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.T( 0.05, 0.07, 100 )

The above function returns a confidence value of 0.013889519

Therefore the confidence interval is 1.8 ± 0.013889519, which is equal to :

1.78110481 to 1.813889519


Confidence.T Function Common Errors

If you get an error from your Excel Confidence.T 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
#DIV/0! Occurs if the supplied size argument = 1
#VALUE! Occurs if any of the supplied arguments are non-numeric

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