Section 3: Numeric Functions 47
Correlation Coefficient. Both linear regression and linear estimation presume
that the relationship between the x- and y-data values can be approximated, to
some degree, by a linear function (that is, a straight line). The correlation
coefficient (r) is a determination of how closely your data it a straight line. The
correlation coefficient can range from r = +1 to r = −1. At r = +1, the data fall
exactly onto a straight line with positive slope. At r = −1, the data fall exactly
onto a straight line with negative slope. At r = 0, the data cannot be
approximated at all by a straight line. With statistics accumulated in registers R
0
through R
5
, the correlation coefficient r is calculated by pressing ´j (or
´ª).
The number that appears in the display will be a ŷ-value (or
x
ˆ
-value) (which is
meaningless if you did not key in a specific x-value (or y-value), as described
above). To view the correlation coefficient value (r), exchange the contents of
the X- and Y-registers by pressing ®.
Example. Using the statistics saved from the previous example, if Voltz wishes
to predict coal production (y) for 1977, she keys in an estimate of electrical
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