Statistical Process Control (SPC) - Statistical evaluation of the output of a process during production
Variations and Control
In-control
Out-of-control
Random variation: Natural variations in the output of process, created by countless minor factors
Assignable variation: A variation whose source can be identified
random variations only
random and assignable variations
The Control Process (Steps to Plot SPC-Chart)
Define: establish control limits
Measure: take a sample and measure the units in the sample
Compare to a standard: compare the sample results against the control limits
Evaluate: If the sample results are within the limits, in-control; or otherwise, out of control, go on to the next step
Take corrective action: investigate the cause of deviation and take corrective actions if necessary
Evaluate corrective action and go back to step 2.
mean (μ)= 0, σ = 1
shift of the mean
increase of standard deviation, σ = 2
Types of Control Charts
Control Charts for Variables - for continuous
measures such as temperature, volume, etc.
Mean Chart (or -Chart): detects shift of the mean
Range Chart (or R-Chart: detects change in shape of distribution
Control Charts for Attributes: for discrete measures such as number of complaints, scratches, etc
p-Chart: measures percent defective
c-Chart: measures # of defects per sample