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Control Charts

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The control chart is a statistical quality control tool used in the monitoring ... of interest is measured on a nominative or an interval, ie: categorical, scale... – PowerPoint PPT presentation

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Title: Control Charts

1
Control Charts
• originally developed by
• Walter A. Shewhart

2
Control Charts
• The control chart is a statistical quality
control tool used in the monitoring variation in
the characteristics of
• a product or service

3
Control Charts
• The control chart focuses on the time dimension
and the nature of the variability in the system.

4
Control Charts
• The control chart may be used to study past
performance and/or to evaluate present conditions.

5
Control Charts
• Data collected from a control chart may form the
basis for process improvement.

6
Control Charts

7
Control Charts
• UCL Process Average 3 Standard Deviations
• LCL Process Average - 3 Standard Deviations

X
UCL
3?
Process Average
- 3?
LCL
TIME
8
Control Charts
• Graph of sample data plotted over time

Assignable Cause Variation
UCL
Process Average ???
Mean
LCL
Random Variation
9
Control Charts
X
X
X
Special Cause Variation two points outside
control limit
Common Cause Variation no points outside control
limit
Downward Pattern no points outside control
limit however, eight or more points in trend
10
Control Charts
• Attribute charts
• Variables charts

11
Control Charts
• Charts may
• be used for categorical variables.
• i.e. attributes

12
Control Charts
• Whenever a character of interest is measured on
a nominative or an interval, ie categorical,
scalean attributes control chart is used to
monitor a process.

13
Control Charts
• Attributes Control Charts
• counts c-chart
• proportion p-charts

14
Control Charts
• Attributes Control Charts
• when sample size are not constant
• and/or are unknown
• use counts charts
• c-charts

15
Control Charts
16
Control Charts
• Attributes Control Charts
• when sample size are constant
and are known
• use proportion charts
• p-charts

17
Control Charts
18
Control Charts
• Charts may
• be used for interval or ratio data i.e.
variables

19
Control Charts
• Whenever a character of interest is measured on
an interval or a ratio scale, a variables control
chart is used to monitor a process.

20
Control Charts
• Variables Control Charts
• Mean and Range charts
• x-bar R charts

21
Control Charts
• Variables control charts are typically used
used in pairs.
• one chart monitors process average while
the other monitors the variation in a process.

22
Control Charts
23
Control Charts
24
Control Charts
• In conclusion
• the control chart is a means of monitoring
variation in the characteristics of
a product or service

25
Control Charts
• Know the assumptions let the computer do the
hard work.

26
Control Charts
• Questions?

27
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(No Transcript)