Module12.ppt Statistical process control

Statistical process control

1. Module 12
Statistical Process Control
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2. Quiz
Accuracy:
The degree of closeness of measurements of
a quantity to that quantity's actual (true)
value.
Precision:
The degree to which repeated measurements
under unchanged conditions show the same
results.
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3. Statistical Process Control (SPC)
•A methodology for monitoring a process to
identify special causes of variation and
signal the need to take corrective action
when appropriate
•SPC relies on control charts
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4. 4
Common
Causes
Special
Causes

5. 5
Histograms do not
take into account
changes over time.
Control charts
can tell us when a
process changes

6. Control Chart Applications
•Establish state of statistical control
•Monitor a process and signal when it
goes out of control
•Determine process capability
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7. Capability Versus Control
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Control
Capability
Capable
Not Capable
In Control Out of Control
IDEAL

8. Commonly Used Control Charts
•Variables data
–x-bar and R-charts
–x-bar and s-charts
–Charts for individuals (x-charts)
•Attribute data
–For “defectives” (p-chart, np-chart)
–For “defects” (c-chart, u-chart)
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9. Developing Control Charts
1.Prepare
–Choose measurement
–Determine how to collect data, sample size,
and frequency of sampling
–Set up an initial control chart
2.Collect Data
–Record data
–Calculate appropriate statistics
–Plot statistics on chart
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11. Next Steps
3.Determine trial control limits
–Center line (process average)
–Compute UCL, LCL
4.Analyze and interpret results
–Determine if in control
–Eliminate out-of-control points
–Recompute control limits as necessary
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16. Typical Out-of-Control Patterns
•Point outside control limits
•Sudden shift in process average
•Cycles
•Trends
•Hugging the center line
•Hugging the control limits
•Instability
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17. Shift in Process Average
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18. Identifying Potential Shifts
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19. Cycles
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20. Trend
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21. Final Steps
5.Use as a problem-solving tool
–Continue to collect and plot data
–Take corrective action when
necessary
6.Compute process capability
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22. Process Capability Calculations
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23. Excel Template
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25. Special Variables Control Charts
•x-bar and s charts
•x-chart for individuals
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30. Charts for Attributes
•Fraction nonconforming (p-chart)
–Fixed sample size
–Variable sample size
•np-chart for number nonconforming
•Charts for defects
–c-chart
–u-chart
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41. Control Chart Selection
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Quality Characteristic
variable attribute
n>1?
n>=10 or
computer?
x and MR
no
yes
x and s
x and R
no
yes
defective defect
constant
sample
size?
p-chart with
variable sample
size
no
p or
np
yes
constant
sampling
unit?
c u
yes no

42. Control Chart Design Issues
•Basis for sampling
•Sample size
•Frequency of sampling
•Location of control limits
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44. Pre-Control
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nominal
value
Green Zone
Yellow Zones
Red
Zone
Red
Zone
LTL UTL

45. SPC Implementation Requirements
•Top management commitment
•Project champion
•Initial workable project
•Employee education and training
•Accurate measurement system
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