Control Charts

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Control Charts

• Using Minitab

Control charts display the process average
(center) and process variability (spread).
Process Average is the expectation and Process
Variability indicates risk associated with the
expectation.
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Frequently Used Control Charts

• Attribute Go/no-go Information, sample size of
  50 to 100
• p-chart (Proportion Defectives)
• c-chart ( Defects)
• u-chart (Defects/Unit)
• Variable Continuous data, usually measured by
  the mean and standard deviation, sample size of 2
  to 10
• X-bar and R-charts
• X-bar and s-charts
• X-charts for individuals

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Attribute Charts in Minitab
Stat / Control charts / Attribute Charts
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p-Chart
Hour SampSize Non- conforming
1 75 17
2 75 18
3 75 18
4 75 16
5 75 20
6 75 22
7 75 24
8 75 19
9 75 15
10 75 22
11 75 19
12 75 24
13 75 12
14 75 22
15 75 20
16 75 20
P charts are used to study the proportion of
defectives in each sample. A defective is an
attribute that makes something useless.
Data on non-conforming (defective) chips produced at a facility
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c-Chart
Unit SqFt Blemishes
1 66 2
2 66 2
3 66 0
4 66 0
5 66 1
6 66 4
7 66 0
8 66 0
9 66 2
10 66 3
Data are number of blemishes in 66sqft of painted area
C charts are used to display the number of
defects in each sample. A defect is a flaw that
is undesirable but does not make the product
useless.
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U-Chart
Unit SqFt Blemishes Rate
1 73 5 0.0685
2 70 4 0.0571
3 60 1 0.0167
4 57 1 0.0175
5 58 0 0.0000
6 57 3 0.0526
7 59 1 0.0169
8 52 4 0.0769
9 55 1 0.0182
10 59 2 0.0339
11 67 0 0.0000
12 55 2 0.0364
13 54 1 0.0185
Data are number of blemishes in different areas

U charts are used to display the number of
defects per unit of measurement. Useful when
samples are of different sizes.
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Variable Charts in Minitab
Stat / Control charts / Variable Charts for
Subgroups or Individuals
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XBar-R Charts
X-Bar charts are used to plot subgroup means.
Sample Obs1 Obs2 Obs3 Obs4 Obs5
1 11.991 12.001 12.176 12.004 12.030
2 12.065 12.018 12.223 11.955 11.959
3 11.946 11.980 12.089 12.125 12.111
4 12.011 12.069 12.065 12.155 12.197
5 11.783 12.140 12.008 12.076 12.044
6 11.962 12.132 12.149 11.991 12.128
7 12.163 11.961 11.932 12.115 11.870
8 12.053 12.053 11.889 11.975 12.037
9 12.108 11.942 12.102 11.967 12.056
R charts are used to plot the range for each
subgroup.
Data represents actual contents in 12 oz soda cans
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XBar-S Charts
X-Bar charts are used to plot subgroup means. S
charts are used to plot the standard deviation of
each subgroup.
Sample Obs1 Obs2 Obs3 Obs4 Obs5
1 11.991 12.001 12.176 12.004 12.030
2 12.065 12.018 12.223 11.955 11.959
3 11.946 11.980 12.089 12.125 12.111
4 12.011 12.069 12.065 12.155 12.197
5 11.783 12.140 12.008 12.076 12.044
6 11.962 12.132 12.149 11.991 12.128
7 12.163 11.961 11.932 12.115 11.870
8 12.053 12.053 11.889 11.975 12.037
9 12.108 11.942 12.102 11.967 12.056
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Is the Process in Control?
XBar-S Chart
X (individual) Chart
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Process Capability
Process capability refers to the ability of a
process to produce a defect-free product or
service in a controlled manner of production or
service environment