Vol. 7, No. 06
from Professor Cleary
Marsha Mallow seems to have a square root in a round hole, or she may need to refer to her cookbook, rather than blurting out a response unassisted.
The use of a sample size of 5 came about primarily for convenience, since it provides an easy way to calculate the mean.
Traditionally, the mean is calculated with the following recipe (formula, that is):
First sample: 75, 85, 75, 85, 65
Following is a shortcut method:
By taking the sum of the samples, multiplying by 2, then moving the decimal point one place to the left, the same result ensues:
Control charts, you will recall, were developed in the 1930s, before the $5.00 calculator or the PC was available for statistical calculations. Clever people developed shortcuts with their minds, rather than with electronic help.
By the way, don't be tempted to confuse your kids by showing them this "new" way to calculate averages. The method works only with a sample size of 5.
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