I had to look this up, its ages since did this.

It is based on the standard error of the mean. Say a sample of 100 items has
a mean of 50 and a standard deviation of 20. The interval estimate with 95%
confidence for the population mean is:

=1.96*std_Dev/SQRT(sample_size) which gives four.
The table below shows the figures and formulas. (I created Range names for
the numbers to make it more clear. Select labels and figures and choose
Insert, Names, Create - make sure the left labels checkbox is ticked and
click OK

sample size 100 100
mean 50 50
std Dev 20 20

Error of mean 3.92 =1.96*std_Dev/SQRT(sample_size)
Mean + Err 53.92 =mean+b5
Mean - Err 46.08 =mean-b5
new N 384.16 =(std_Dev*1.96/2)^2

so to reduce the error of the mean by half we increase the sample size by:
=(std_Dev*1.96/2)^2

of course you have to get you mean and stdev first or make an estimate of
them first.

Hope this helps
Peter

"Ariel" wrote:

Can someone give me the spreadsheet formulas for calculating a sample size
for independent samples?

Thanks

Ariel