1. Statistical Process Control
Module 12
Statistical Process Control

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.

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

4. Common Causes
Special Causes

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

7. Capability Versus Control
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)

9. Developing Control Charts
Prepare
Choose measurement
Determine how to collect data, sample size, and frequency of sampling
Set up an initial control chart
Collect Data
Record data
Calculate appropriate statistics
Plot statistics on chart

11. Next Steps Determine trial control limits
Center line (process average)
Compute UCL, LCL
Analyze and interpret results
Determine if in control
Eliminate out-of-control points
Recompute control limits as necessary

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

17. Shift in Process Average

18. Identifying Potential Shifts

19. Cycles

20. Trend

21. Final Steps Use as a problem-solving tool Compute process capability
Continue to collect and plot data
Take corrective action when necessary
Compute process capability

22. Process Capability Calculations

23. Excel Template

25. Special Variables Control Charts
x-bar and s charts
x-chart for individuals

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

41. Control Chart Selection
Quality Characteristic
variable
attribute
defective
defect no
n>1?
x and MR
constant
sampling
unit?
yes
constant
sample
size?
yes
p or np no
n>=10 or
computer?
x and R
yes no no
yes
p-chart with
variable sample
size
c u
x and s

42. Control Chart Design Issues
Basis for sampling
Sample size
Frequency of sampling
Location of control limits

44. Pre-Control
nominal
value
Green Zone
Yellow Zones
Red
Zone
LTL
UTL

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