How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

It is a fixed value, IF you are dealing with a range calculation taking two points at a time. That is basically what the "moving range" is - it is the average value for the equivalent of a xbar-R chart of group size 2. The 2.66 includes the conversion from the expected value of the average range to the expected value of the standard deviation (which is an extremely tough calculation figured out and tabulated in the early 1900's), and includes multiplying by 3 for three standard deviations from the average.

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Thanks.

but i read some ppl calculated with 1.128?

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

3 divided by 1.128 = 2.66

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

oh ok...great. i got it. =)
thanks.

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Hi All,
Also can any one help me do draw Specification limits using below data. Can i use X-MR for the same. I dont have any timelines for below this just a measure obtained after a review of 8 project

Project 1 - 25 Defect
PRoject 2 - 15 Defect
Project 3 - 35 Defect
Project 4- 12 Defect
Project 5 - 5 Defect
Project 6 - 22 Defect
PRoject 7 - 32 Defect
PRoject 8 - 29 Defect

Thanks
KP

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

kotamballi said:

Hi All,
Also can any one help me do draw Specification limits using below data. Can i use X-MR for the same. I dont have any timelines for below this just a measure obtained after a review of 8 project

Click to expand...

Perhaps this is a language issue, but if you really mean specification limits they come from what is acceptable to you (or your organization); and so they are not calculated from the actual process results.


If you actually mean control limits (which is the subject matter of this part of the forum) then I would actually recommend using the c chart, not the X, MR chart. The X, MR is meant for continuous data while the c chart is meant for categorical - or defect data. The formula for the c chart is fairly straightforward: Cl = average defect count + 3*sqrt(average defect count)

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Bev
Thanks for the reply.. Yes I meant Control limits... Will try to use this. Thanks a lot..
Thanks
KP

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Hi, please help to clarify on whether the way i am calculating the control limits are correct.
I have 128 data (with 4 subgroups-each subgroup represents 32data) and the following will be the step that I took to calculation the control limits for X-Bar: -
1. Determine an average, mean (x-bar)
2. UCL = Average+(Average/6)x3
3. LCL = Average-(Average/6)x3

Can I use the same set of data (128 data with 4 subgroups) to calculate the range control limits?
Can I use the same method to calculate the control limits?
Thanks.

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

those are not the formulas for the control limits on the average. I'm curious as to where you got those formulas?

do you have categorical (pass/fail or count) data or do you have continuous (measurement) data? the type of data will dictate the formula.

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

The PERT Chart (which is NOT a control chart) uses 1/6 of the range of times in the schedule as an estimate of the standard deviation. This would NOT be appropriate for SPC.