Vol. 10, No. 8

August 2008

PQ Systems

A picture is worth...

Quality Quiz: With a video!

Data in everyday life

MSA with Jackie Graham

Bytes and pieces

FYI: Current releases


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Quality Quiz from Professor Cleary

"C" is correct.

Click here for a more complete video explanation

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.

Traditional method:

For example:

Sample: .65, .70, .65, .65, .85

Shortcut method:

Taking the sum of the samples, multiplying by 2, and moving the decimal point one place to the left generates the same result:

.65 + .70 + .65 + .65 + .85 =3.5 x 2 = 7.0

Moving the decimal point one place to the left creates .70.

Remember that the control chart was developed in the 1930s before the availability of the ten-dollar calculator or the PC for statistical calculations. Conventions such as this rendered calculations easier and more convenient.

By the way, don’t confuse your kids by announcing this “new” way of calculating averages. The method works only with a sample size of 5.

* Shewhart, W.A., Economic Control of Quality of Manufactured Product (New Jersey: VanNorstrand Reinhold Company, Inc., 1931)


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