I think something like this

=AVERAGE(A2:A20) +CONFIDENCE(0.05,STDEV(A2:A20),COUNT(A2:A20))
=AVERAGE(A2:A20) -CONFIDENCE(0.05,STDEV(A2:A20),COUNT(A2:A20))

See help on CONFICENCE

HTH,
Bernie
MS Excel MVP


"Vagus" wrote in message
...
hello
I have a problem in calculating the uncertainty (95% ci) of particulate mass
concentration. The weighings of the PM mass have been done 3 times in
different days under same conditions.
I hope you have the solution.