Any Statistics folks out there?

[Raptor]

I have not done statistics in 4 years and I am having difficulty understanding where a number comes from:

A soda company bottles soft drinks into 16oz bottles. The overall process average is 16.1oz, which was found by taking a large number of samples, with each sample consisting of 5 bottles. The average range of the process sampleis .25oz. Using 3σ control limits, calculate the c-bar chart upper limit and lower limit control limits for the process.

From this I got:
mean = 16.1
range = .25

I know for the upper control I use:
UCL=Mean + (A)(Range)
UCL=16.1+(.577)(.25)
resulting in 16.244oz. which means the sample is in control.
What I do not understand is what A stands for or how I got it. It has something to do with sigma which I am just missing entirely.

[Raptor]

ok nevermind I got it...
A2 = constant to provide three-sigma limits for the sample mean

So I had to pull it from a chart.

[Jayhawk]

Yup, exactly what I was going to say. 



 

[Banshee7]

Man, you beat me by half a second...