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