Statistical Quality Control

1

• Statistical Quality Control
• Quality means fitness
  for use
• Quality improvement means systematic
• 
  elimination of waste
• Variability
• Chance cause gtgtgt process is in statistical
  control
• Assignable cause gtgtgt process is statistically
• 
  out-of-control
• Objective Quickly detect the occurrence of
• assignable cause
• SPC Statistical Process Control

IME 301
2
3 ( Shewhart) Control Charts
3 gtgtgt 0.0027 and CL 99.73
UCL
3
2
CL
2
3
LCL
t
Upper Control Limit 3 Central Line
Lower Control Limit - 3
IME 301
3
Decision rules for detecting nonrandom patterns
on 3-sigma control chart 1- One point plots
outside 3-sigma control limits. 2- Two out of
three consecutive points plot beyond a
two-sigma limit. 3- Four out of five
consecutive points plot at a distance of
one-sigma or beyond from the center line. 4-
Eight consecutive points plot on one side of the
center line. Source Applied Statistics
and Probability for Engineers by Montgomery and
Runger. Cyclic behavior Trend
IME 301
4
Sample Size and Frequency Different points of
view a) Reliable detection of assignable cause
gtgtgt collect large samples and very
frequently b) Cost of the process control gtgtgtgt
collect small samples in shorter intervals
or collect large samples in
longer intervals
5

Actual state of process Only chance
Assignable
causes causes
Process out False Correct of
control alarm decision Decision
Process in Correct Failure to
control
decision detect
IME 301