The Economics of Poka-Yoke Devices

CAVEAT SURFERLet the web visitor beware. All economic models are based on assumptions that may not hold for you. Use the concepts below at your own risk.

Economic Analysis of Source InspectionsStarr Breakeven Analysis (focuses on fixed cost)In the most recent edition of his operations management text, Martin Starr has a chapter supplement on poka-yoke. In it, he suggests a simple cost evaluation model. It compares the cost of poka-yoke devices with the cost savings from defect elimination. The variables used are defined below:

  • p = the probability of a particular defect occurring
  • V = the daily output volume
  • Cdef = The cost of the defect
  • Cdev = The daily cost of the mistake-proofing device (the total cost of the device divided by payback period expressed in days)

The daily defect cost = p(Cdef)V.

Any device that costs less than the daily defect cost, Cdev< p(Cdef)V, should be seriously considered for implementation.

Analysis of Self-Checks or Successive-checks. Shigeo Shingo was one of the industrial engineers at Toyota who has been credited with identifying setup time reduction as a step in improving manufacturing flexibility and efficiency. Shingo was also largely responsible for creating and promoting self-checks and successive checks (these are explained in the tutorial).

Both self-checks and successive checks gather information about the process by inspecting process outputs. Effective poka-yoke devices reduce the cost of inspection, making it economical to increase both the frequency and quantity of inspection. I

f the cost can be reduced enough, 100% inspection may become economically viable.

Thus poka-yoke devices may potentially have an effect on statistical process control (SPC) analogous to the effect setup time reduction had on the frequency and length of production runs in manufacturing.

Attribute Charts: Same as an np-chart, lower inspection cost to near zero to make it more economical. Analytic model is quite complicated. If a self-check is desired, minimize inspection cost. If possible, create a source inspection instead.

Measurement Charts: Use to sort defects when cost of in-house quality problems is much less than cost of problems at the customers’ facility. If a self-check is desired, don’t expect it to reduce process variation (This issue is discussed in Failsafing and Measurement control charts). Make sure to minimize inspection cost. If possible, create a source inspection instead.