Confidence Intervals and Charts

I wonder if it can be done....

At the moment I have a set of control charts with confidence intervals set at 95% (1.96 times standard deviation). What I'd like to do is for the user to be able to set a confidence interval, say 80% and for this to be automatically shown on the chart.
Plotting it to the chart is the easy bit(!) but does anyone know if this can be done - some sort of formula to amend the multiplier based on the Normal Distribution based on the confidence interval required.
Thanks

Robert

You mean for 95% you should get 1.645. This is possible by using the worksheet function:
=NORMSINV(95%)

- Mangesh

Quite correct - 1.645 for a one-tailed test, 1.96 for a two-tailed test. I can get that to work - thanks very much for the help.
Robert

Ok. for a two tailed test, you woild need to modify the input arg to the function something like this:
If cell A1 is the input cell which is 95% then use
=NORMSINV((1-A1)/2+A1)
for the 2-tailed test

- Mangesh