Spencer -

Here's part of my reply to a related question on Feb. 4, 2006:

The CONFIDENCE worksheet function "gives the half-width of a confidence
interval. It is based on the normal distribution, and it is appropriate when
the population standard deviation is known, i.e., when the standard
deviation does not have to be estimated from the sample data.

If sample data are used to estimate standard deviation, the confidence
interval should be computed using the t distribution, not the normal
distribution. According to the book "Data Analysis Using Microsoft Excel:
Updated for Office XP" by Michael R Middleton, the half-width is

=TINV(1-Conf_Level,n-1)*STDEV(Data)/SQRT(n)
....

What does it mean?

The half-width and the sample mean are used to construct a confidence
interval from (mean - half-width) to (mean + half-width). Referring to the
Middleton book, here are two interpretations.

Subjective Probability Interpretation: Based on the sample there is a 99%
chance that the population mean is between (mean - half-width) and (mean +
half-width).

Relative Frequency (classical statistics) Interpretation: If we repeatedly
selected random samples of size 164 from this population and constructed a
confidence interval for each sample, approximately 99% of the confidence
intervals would contain the true population mean."

- Mike
www.mikemiddleton.com

"sjvicker" wrote in
message ...

I was recently marked down on a lab project for using the excel function
to find the confidence interval for 95% certainty. I properly used the
function but retrieved a different answer than my professor who used
Descriptitive Statistics under the Data Analysis-Tools feature.

The only way I can recover the points is to figure out why the answers
were different.

The sample size is 20, Mean 25.87, Max Value 28, Min Value 24.2,
Standard Deviation 1.01, Range 3.80, Mode 26 and Median 25.75

Any ideas why our values were different? did excel possibly use
different tables?

-Spencer