What Taguchi Loss Function has to do with Cpm?

    Amrendra Roy

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    The traditional way of quality control can be called as “GOAL-POST” approach where, the possible out-come is goal or no-goal. Similarly, QA used to focus only on the end product’s quality with two possible outcomes, pass or fail.


    Later on Taguchi gave the concept of producing products with quality targeted at the center of the customer’s specifications. He stated that as we move away from the center of the specification, we incur cost either at the producer’s end or at the consumer’s end in the form of re-work and re-processing. Holistically, it’s a loss to the society.


    For example;

    Buying a readymade suit, it is very difficult to find a suit that perfectly matches your body’s contour, hence you end up going for alterations. This incurs cost. Whereas, if you get a suit stitched by a tailor that fits your body contour (specification), it would not incur any extra cost in rework.

    Let’s revise what we learned in “car parking” example (see links below). The Cp only focuses on how far the process control limits (UCL & LCL) are from the customer’s specification limits (USL & LSL) …. it doesn’t take into the account the deviation of process mean from the specification mean. Hence, we  require another index which can penalize the Cp for the above deviation and this new index is called as Cpm.

    Related Posts

    Why & How Cpm came into existence? Isn’t Cpk was not enough to trouble us?

    Car Parking & Six-Sigma

    What’s the big deal, let’s rebuild the garage to fit the bigger car!

    How the garage/car example and the six-sigma (6σ) process are related?

    Now Let’s start talking about 6sigma

    What do we mean by garage’s width = 12σ and car’s width = 6σ?

    Kindly provide feedback for our continuous journey

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