Example of Variation

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    Dave Woodmore

    I’m looking for ideas to illustrate to a group, the concept of common cause and special cause variation.
    I reckon something that takes less than 5 minutes, is practical (everybody can have a go) and costs nothing.
    Any thoughts?


    Robert Butler

    Before running this experiment make sure that your watch is stopped at a time about an hour or two before the actual running of the test.
    Have an overhead ready in advance and then announce to everyone in the room that, on your mark, they should write down the time (to the nearest second) that is displayed on their respective wristwatches.  Go around the room and ask each person in turn for their recorded time.  Save your “stopped watch” time until last.  The variation about the actual current time will illustrate common cause variation and your stopped watch will provide an example of special cause variation…after the exercise is over you will help drive home the concept of special cause if you admit that you stopped your watch on purpose.


    M Schaeffers

    You only need a white board. This will explain common and special cause variation and the use of control limits and spec limits in 5 minutes.
    Draw a road, the middle and the side of the road are your spec limits. You draw a carr which is not exactly in a straigt line but there is some variation and you have your wheels to correct for it. You don’t respond to the spec limits but you try to keep your car on a specific target.
    If your varioation suddenly becomes much bigger (bad road, more wind, you can ask people for examples and you will definetely get drunk driving) these are assignable causes. This means you have much more variation and you should either stop and look for the cause (flat tire) or adjust your speed (bad road, more wind etc).
    Nobody keeps driving 80 miles per hour if the variation is too large and tries to keep the car on the road by only adjustong the wheels, why do we don’t apply the same practice in manufacturing ?
    Hope this helps.



    Ask the students to write 5 capital letter “A”s. Next, instruct them to write 5 more As, this time using the other hand.
    Here’s how you explain it:
    Imagine you are a company producing capital letter As. How long have you been producing these As, or in other words, how long have you been writing these As? The estimate should be around 20 to 25 years (depending on your audience).
    Now tell them, they have been writing the As for so long and yet, the first set of As are not perfectly alike (but they do look similar). This is because of common cause variation. You can not eliminate them, but you can improve on them.
    Now take a look at the next set of As. Do you think the difference is attributed to common cause? Of course not. This is what you call special cause variation. What was the assignable cause ? It’s the changing of writing hand.
    Hope this helps.

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