Vol. 8, No. 10

October 2006

PQ Systems

GAGEpack in Los Alamos National Laboratory

Quality Quiz: With a video!

Six Sigma

Data in everyday life

Bytes and pieces

FYI: Current releases


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Quality Quiz:

Click here for a more complete video explanation

When Kohl Minor tried to explain coefficient of determination to his boss, Hal Zangels, last month, he used the data below, referring to tons of coal and total cost for his mine.

Kohl used CHARTrunner's scatter diagram to analyze the data, making total cost the dependent variable (Y) and total tons produced, the independent variable (X). This generated the following chart:

The regression line in simple linear regression is a straight line drawn through the points plotted on the scatter diagram. The calculation results in an equation where the value of Y (on the vertical axis) can be predicted from the value of X (on the horizontal axis). The equation takes the form of

Last month, Kohl explained to his boss, Hal Zangels, that the coefficient of determination represents the ratio of explained variance of Y to the total variation of Y. In this case, it is 0.76. But Hal pressed Kohl for further explanation. He had memorized the definition from a textbook without fully understanding its application.

To further assure Hal of his competence, Kohl added, "Well, if you take the square root of this number 0.76^2, you will have the correlation coefficient, which is 0.87. And of course you know what that means."

Analyze Hal's competence for yourself, and predict the scenario to follow:

a) Hal is convinced that Kohl understands coefficient of determination, since he is correct in this assertion;

b) Kohl has dug his own statistical grave, not only with his sarcastic remark but also the demonstration of his lack of comprehension of the concept.

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