The Elsmar Cove Forum How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL
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#1
12th October 2006, 11:06 AM
 maxfusion Posts: 5
How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL

Hi all

how do i calculate the UCL,LCL & CL for the below data. it has a total of 30 datas and i need to find out also the formula for UCL,LCL & CL as i need to use excel to do the calculations. PLS HELP ME!!!

Tensile strength
2.1
2.5
3.1
3.2
2.3
1.9
2.1
2.2
2.5
3.1
1.8
2.2
3.2
1.4
1.6
2.0
3.1
2.5
3.6
3.3
2.3
2.1
1.9
1.4
1.5
2.5
3.1
4.0
3.5
3.2

#2
12th October 2006, 12:31 PM
 Bev D Posts: 2,960
Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

calculate the moving range between each value: MR1 = the absolute absolute value of the second value - first value. MR2 = the absolute absolute value of the third value - second value and so on. you will have 29 of these values.
then calculate the average of these 29 values. this is the average moving range, MR Bar

The CL = is the average of the 30 readings.
LCL = average - 2.66*MRbar
UCL = average + 2.66*MRbar

this will ONLY work if the tensile values are in order of PRODUCTION of hte parts. if you dont' know the order of production and you only know the order of the tensile measurements you cannot do the control chart. teh dat must in production time sequence.

But I have to add that I am concerned (no offense intended) that this is a very basic question that can be readily answered by a simpel google search or any SPC book that is lying around. You provide very little background as to the nature of the problem. You seem to have almost no knowledge of SPC and I wonder why you are tasked - feel tasked - to do this on your own (my assumption since you're here)?
 Thanks to Bev D for your informative Post and/or Attachment!
#3
12th October 2006, 02:33 PM
 preetham - 2007 Posts: 52
Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

I would like put some comment on this,

Just calculating the UCL & LCL will not tell you that the process is capable of producing the parts per customer requirement.

The UCL,LCL & CL only tells you that the readings you have got are in statistically under control or Not,you have to calculate the Process capability & index then only you will find out that the obtained values are good enough to satisfy the customer specification.CP & CPk value tells that the process is capable of producing the parts per the customer specification .
#4
12th October 2006, 03:29 PM
 Bev D Posts: 2,960
Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

of course the LCL and UCL do not provide an assessment fo capability. But, the OP had a very limited and specific question. I answered that question but only after thinking about it for awhile as the OP had obviously not researched the quesiton on their own and provided so little information on his issue that any answer we could give would be highly likely to be mis-used.
#5
13th October 2006, 12:41 PM
 maxfusion Posts: 5
Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

my apologies if mine question is not through enough.

i actually want to draw a x-bar control chart using the data that i have
but i just do not know the formula and what should i use for the variables

i would like to know if it is possible to draw a x bar control chart for individual measurement as all 30 datas are individual

Last edited by maxfusion; 13th October 2006 at 12:59 PM.
#6
13th October 2006, 01:51 PM
 Bev D Posts: 2,960
Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

The formulas I described in my original response ARE the formulas you are looking for.
#7
14th October 2006, 01:04 AM
 maxfusion Posts: 5
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
#8
8th January 2007, 05:08 PM
 Marc Posts: 24,266
Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Quote:
 In Reply to Parent Post by maxfusion How does 2.66 comes about because 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

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