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            Measure of association between two variables, equal to the sample 
            covariance divided by the product of the sample standard deviations 
            of X and Y. A correlation of -1 indicates a perfect inverse 
            relationship, 0 indicates no relationship, and +1 indicates a 
            perfect direct relationship. The formula for the correlation 
            coefficient is:  
            
               
            
            
              
            
            
              
            
            
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