Wolfram Research
Wolfram Math World: Spearman Rank Correlation Coefficient
This page gives a description of the Spearman Rank Correlation Coefficient, which is an approximation to the exact correlation coefficient, r. It is somewhat easier to compute than the exact correlation coefficient. Also provides links...
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Correlation Coefficients
This page provides a good discussion of correlation coefficients (r,) which indicate the "strength of the linear association between variables." Includes formulas and scatter plots. Some prior knowledge of statistics might be helpful.