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8/9/2019 Chapter 8B Qualtiy Control
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ncs3x
n
c
s
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becomelimitscontrolthat theSoc
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:isdeviationstandardprocesstheofestimateOur
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8/9/2019 Chapter 8B Qualtiy Control
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sAx)xLCL(
sAx)xUCL(
nc
3
ALetting
3
3
43
!
!
!
Where the value for A3 depends on subgroup size.
8/9/2019 Chapter 8B Qualtiy Control
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nc
s2xBandAzonesupperbetweenBoundary
ncs2xCandBzonesupperbetweenBoundary
nc
s-xCandBzoneslowerbetweenBoundary
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8/9/2019 Chapter 8B Qualtiy Control
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An Example
y
In a converting operation, a plastic film is combined withpaper coming off a spooled reel.
y As the two come together, they form a moving sheet thatpasses as a web over a series of rollers.
y
The operation runs in a continuous feed, and thethickness of the plastic coating is an important productcharacteristic.
y Coating thickness is monitored by a highly automated
piece of equipment that uses 10 heads to take 10measurements across the web at half-hour intervals.
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Head # 8:30 9:00
1 .08 .1
.26 2.02
3 2.13 2.1
1.9 1.9
5 2.30 2.30
6 2.15 2.08
7 2.07 1.9
8 2.02 2.12
9 2.22 2.15
10 2.18 2.36
ve. 2.1 2.120.111 0.137
17:00 17:30 18:00
1.98 2.08 2.22
2.30 2.12 2.05
2.31 2.11 1.93
2.12 2.22 2.08
2.08 2.00 2.15
2.10 1.95 2.272.15 2.15 1.95
2.35 2.1 2.11
2.12 2.28 2.12
2.26 2.31 2.10
2.18 2.1 2.10
0.121 0.113 0.106
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There are no indications of a lack of control, so the process can
be considered to be stable and the output predictable withrespect to time, as long as conditions remain the same.
8/9/2019 Chapter 8B Qualtiy Control
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y It is now appropriate to use some of the methods that will be
described in Chapter 10 (such as check sheets, Paretoanalysis, or brainstorming) to attempt to reduce the common
causes of variation in the never-ending quest to decrease the
difference between process performance and customer
needs.
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Individuals and Moving Range Charts
It is not uncommon to encounter a situation where only asingle variable value can be periodically observed forcontrol charting.
Perhaps measurements must be taken at relatively long
intervals, or the measurements are destructive and/orexpensive; or perhaps they represent a single batchwhere only one measurement is appropriate, such as thetotal yield of a homogenous chemical batch process.
Whatever the case, there are circumstances when datamust be taken as individual units that cannotconveniently be divided into subgroups.
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Individuals and moving range charts have two parts:
y
One charting the process variabilityy One charting the process average
The two parts are used in tandem much as the
and R chart. Stability must first be established in
the portion charting the variability, because theestimate of process variability provides the basis for
control limits of the portion charting the process
average.
x
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Single measurements of variables are considered a subgroup ofsize one.
Hence, there is no variability within the subgroups themselves,and an estimate of the process variability must be made insome other way.
An estimate of variability is based on the point-to-point
variation in the sequence of single values, measured by themoving range (the absolute value of the difference betweeneach data point and the one that immediately preceded it):
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1-ii R !
An avera eofthe moving ranges isusedas
the centerlineforthe moving rangeportion
ofthe chart andasabasisofan estimateof
theoverallprocessvariation:
1-k
RRRange)(MovingCenterline
!!
Where k is the numberofsingle measurements.
As it is impossible to calculate to moving rangeforthefirst subgroupbecause noneprecede it, thereareonly k-1
range measurements;so thesum ofthe Rvalues isdivided
by k-1.
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8.6Individuals and Moving Range
Charts
For the individuals portion of the control chart, thecenterline is the average of the single measurements. Wefind control limits by adding and subtracting three timesthe standard deviation of the single measurements,
estimated by Rbar/d2:
Centerline(x) = xbar = x/k (8.54)
UCL(x) = xbar + 3(Rbar/d2) (8.55)
Using the factor E2 to represent 3/d2, the expression forthe upper control limit becomes
UCL(x) = xbar + E2Rbar (8.56)
Where E2 depends on subgroup size.
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Forthe individualsportionofthecontrolchart,the
centerline istheaverageofthesinglemeasurements.
Wefindcontrollimitsbyaddingandsubtractingthree
timesthestandarddeviationofthesinglemeasurements,
estimatedby:d
R
si e.subgoupondependsEWhere
RE)(U L
dforEfactorthengUsi
d
R)(U L
k( )enterline
2
2
2
2
2
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In this case the subgroup size is 2, as we use two
observations to calculate each moving range value.
Hence, E2 = 2.66, and
R2.66x(x):usingfoundisli itcontrollowertheSi ilarly
R2.66x(x)
!
!
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y A chemical company produces 2,000-gallon batches of a chemicalproduct, A-744, once every two days.
y The product is a combination of six raw materials, of which threeare liquids and three are powdered solids.
y Production takes place in a single tank, agitated as the ingredientsare added, and for several hours thereafter.
y Shipments of A-744 to the customer are made in bins as single lotswhen the batches are finished.
y The chemical company is concerned with the density of thefinished product, which it measures in grams per cubic centimeter.
y As batches are constantly stirred during production, the density is
assumed to be relatively uniform throughout each batch.y Therefore, management decides that density will be measured by
only one reading per batch. During a 60-day period, 30 batches ofA-744 are produced.
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Date Density Moving
Range
Date Density Moving
Range
5/6 1.2 2 - 6/10 1.253 0.018
5/8 1.289 0.0 7 6/12 1.257 0.00
5/10 1.186 0.103 6/1 1.275 0.018
5/13 1.197 0.011 6/17 1.232 0.0 3
5/15 1.252 0.055 6/19 1.201 0.031
5/17 1.221 0.031 6/21 1.281 0.080
5/20 1.229 0.078 6/2 1.27 0.007
5/22 1.323 0.02 6/26 1.23 0.0 0
5/2 1.323 0.000 6/28 1.187 0.0 7
5/27 1.31 0.009 7/1 1.196 0.009
5/29 1.299 0.015 7/3 1.282 0.0865/31 1.225 0.07 7/5 1.322 0.0 0
6/3 1.185 0.0 0 7/8 1.258 0.06
6/5 1.19 0.009 7/9 1.261 0.003
6/7 1.235 0.0 1 7/11 1.201 0.060
Totals 37. 98 1.087
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The process appears to be in a state of statistical control, since
there are no points beyond the control limits and no other
signs of any trends or patterns in the data.
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Special Characteristics ofIndividuals
and Moving Range Charts
Because each subgroup consists of only one value, andprocess variation is estimated on the basis of observation-to-observation changes, individuals and moving range
charts have certain unique characteristics that distinguishthem from other control charts.
For example, for an individuals and moving range chart tobe reliable, it is best to have at least 100 subgroups,
whereas 25 will suffice for most other control chartforms.
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Correlation in the Moving Range.
The moving ranges tend to be correlated.
Large moving range values tend to be followed by other largemoving range values tend to be followed by other largemoving range values, and small moving range values tend to
be followed by other small moving range values.
Because of this, users must be cautious in applying rules for alack of control dealing with patterns in the data.
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It is usually best to be conservative when applying the rules
concerning patterns in the data other than points beyond the
control limits that indicate a lack of control in moving range
charts.
For example, instead of 8 consecutive values above or below
the centerline indicating a lack of control, we might require
10 or 12.
Knowledge and experience are the best guides in establishing
policy in this case.
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Inflated Control Limits.
The control limits for individuals and moving range charts arecomputed from individual measurements.
One indication that the control limits are inflated is the
occurrence of at least of the data points below the
centerline of the moving range portion of the control chart.
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When this happens, control limits should be based on
the median of the moving range values, rather than
those based on the average of the moving Range
values. is calculated using the following
procedure:
1. Arrange the subgroup ranges from low to high.
2. If there are an odd number of subgroup ranges, select the middle
subgroup range as the median range.
3. If there are an even number of subgroup ranges, select the two
middle most subgroup ranges and compute their average. Thisaverage is the median range.
ReM
ReM
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Ifinflated control limits are suspected, control limits
based on the median of the moving ranges should becalculated and compared to those based on the averagemoving range; the narrower of the two sets should beused. Control limits for the moving range portion,based on the median, can be computed using:
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0.000range)movingLCL(median
M3.865range)movingUCL(median e
!
!
(Again, recallthatthe assumption ofnormality is notrequired to interpretthe controllimits;the Empirical
ule maybe used instead).
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Controllimitsforthesinglemeasurements
portionarecreatedusingthefollowing
equations:
e
e
M.CL(x)
Similarl
M.xCL(x)