Interpreting X bar and R Control Charts

M

Muhd Firdaus

#1
Hi,
i am looking for clear SOP for interpreting the X bar & R Chart. Can anyone provide me with clear guideline on how to interpret them. There are 4 terms which are commonly used (RUN,TREND,PERIODICITY,HUGGING) and i am not familiar with them at the moment. Kindly assist me. Thank You
 
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#2

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#3
Hi,
i am looking for clear SOP for interpreting the X bar & R Chart. Can anyone provide me with clear guideline on how to interpret them. There are 4 terms which are commonly used (RUN,TREND,PERIODICITY,HUGGING) and i am not familiar with them at the moment. Kindly assist me. Thank You
I'll take a stab at this, though some authors use different variations. Personally, I prefer to just lump everything into "do I have a signal that something is changing" with a preference towards the use of the word "trend" but some folks here disagree.

RUN - usually a pattern of points one side of the average line, such as a "run of 7 in a row above average". Could also refer to a number of standard deviations from the average, such as a run of 4 in a row one standard deviation above average

TREND - usually a pattern of continually increasing or decreasing points such as "7 in a row all increasing"

PERIODICITY - some form of cycle, the seasonal 12 month cycle being most "famous". Usually more of a judgement call that a specific "rule". Be careful to have at least 5 to 7 cycles before declaring a periodicity is occurring.

HUGGING - the points stay very close to the average line, and do not spread out to the control limits. There are some signal rules based upon number of points in a row not exceeding one standard deviation from the average line.
 
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