Exponentially Weighted Moving Averages

Hi folks,

This may be more of a stats problem than an actual excel fix but here goes! I'm trying to construct a control chart that plots the EWMA of a range of PPM numbers. My problem here is trying to get the control limits to make sense. I've used the mean (or in this case the EWMA) +/-2sigma for the upper and lower controls. The problem is the control limits hug very close to the distribution. Should I be using another mean? Also i've found 2 versions of the formula to calculate the EWMA (as shown in the example book) One of these gives me a range which is identical to me lower control limit. How can this be????!!!

Any help with this would be appreciated!

Cheers,
Manolis