Data correlation

Correlation coefficient r

  The quantity r (linear correlation coefficient) measures the
  strength and the direction of a linear relationship between 
  two groups of data.  The mathematical formula for r is:



  The value of r is:  -1 ≤ r ≤ +1

  
 Positive correlation:  If x and y have a strong positive 
  linear correlation, r is close to +1.  In this case, as
  values for x increase, values for y also increase.
  
 Negative correlation:  If x and y have a strong negative
  linear correlation, r is close to -1.  In this case, as
  values for x increase, values for y decrease.
  
 No correlation:  If there is a weak or no correlation between
  x and y, r is close to 0.  In this case, the relationship 
  between x and y is random, nonlinear.

  A correlation is described as strong when |r| > 0.8,
  weak when |r| < 0.5.

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