Vol. 3, No. 8
from Dr. Cleary
A sample size of 5 allows for a shortcut to
calculate the average. Five is the conventional sample size, not because of its
link to any statistical principle, but because, as statistician Walter Shewhart
pointed out, it makes calculations more convenient.
The use of a sample size of 5 came about because it allowed for an easy way to calculate the mean.
The control chart, remember, was developed in the 1930's before the ten-dollar calculator or the PC were available for statistical calculations.
By the way, don't confuse your kids by announcing this "new" way to calculate the average. This method works only if the sample size is 5.
* Shewhart, W.A., Economic Control of Quality of Manufactured Product (New Jersey: VanNorstrand Reinhold Company, Inc., 1931)
Copyright 2001 PQ Systems.
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