**Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?**
How does 2.66 comes about beacuse in some books they were using d2 as 1.128 under Chart for RANGES. does your 2.66 A3 comes under chart for AVERAGE. please enlighten me.

Thanks

First, the values in the tables are made for Xbar-R charts. The intent of the n=2 two row is that you are plotting the range values (the first range value is the difference between points 1 and 2, the second range value is the difference between points 3 and 4, etc), and you are plotting the Average of each sequential pair on the xbar chart.

In a x-individuals with moving range, the control limits are NOT the same as xbar-R n=2. The control limits for a xbar n=2 chart are:

xbarbar +/- (3 Rbar)/(d2 x sqrt

)

But we don't want the square root of n, since we are not plotting xbars of pairs.

So, the control limits should be:

xbarbar +/- (3 / d2) times the Average moving range.

3/d2 = 3 / 1.128 = 2.66 which is otherwise known as A3