Significance of LCL inPchart (Attribute Chart)

This topic has 5 replies, 6 voices, and was last updated 17 years ago by Tim F.

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November 17, 2004 at 4:54 pm #37569

Pandit
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I would like to know the significance of LCL in the Pchart since the point just above or below the LCL means the same and Series of points moving away from centreline towards Zero is alwas preferable.

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November 17, 2004 at 5:29 pm #110870

Darth
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LCL is truncated to zero because you can’t have less than zero percent defective. Direction to zero is desired.

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November 17, 2004 at 6:44 pm #110875

mjones
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Pandit-
Presuming the LCL of your p-chart is above 0, and movement toward 0 is desired… What does it mean if you have a point “out of control” below the LCL? This point is significantly different! Something happened (probably) that caused this good thing to occur!
This is good news! An opportunity! Go find out what caused the process to be significantly better. Keep doing this and make things better.

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November 17, 2004 at 6:47 pm #110876

SPC junkie
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Points below the LCL can represent one of three conditions:
(1) A legitimate trend toward a new state of control at a lower level of defects (as indicated by Darth)
(2) Deficient inspection, which is finding less defects than statistically expected.
(3) Lack of randomness in the inspected sample

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November 20, 2004 at 4:04 pm #111037

Sankar
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Then Why should I calculate LCL as anything towards ‘0’ is prefered.

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November 20, 2004 at 4:42 pm #111042

Tim F
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LCL (or UCL) is calculated because that is how the system IS behaving. It has nothing to do with how you WANT the system to behave. LCL & UCL (control limits) are “the voice of the process”. Once you know the current behavior, you can watch for changes. The spec limits are “the voice of the customer”. That is want you want. You need to compare what you have (control limits) to what you want (spec limits).
Tim F

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