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Correlation Coefficient
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The degree of correlation (or association) between a series of values x & y is measured by the correlation coefficient, denoted by the letter R.
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The correlation coefficient is measured on a scale +1 through zero to -1. Complete correlation can be +1 or -1.
Any correlation between 0 and 19% is considered very weak, between 20 & 39% is weak, between 40-59% is moderate, 60-79% is strong and 80-100% is very strong.
For any set of numerical values x and y in ranges x & y, the Correlation coefficent can be calculated as follows.
R = Σ(x*y-n*Mean(x)*Mean(y))/((n-1)*SD(x)*SD(y))
Where Mean (x) & Mean (y) are the averages of x & y respectively.
SD(x) and SD(y) are the standard deviation values of x & y respectively.
n is the number of values.
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