A correlation is an association / relationship between the occurrence of two or more events, a correlation coefficient is a signed number signifying the strength & direction of that relationship.

       Perfect positive correlation coefficient (+1.00) (always)
        Positive correlation coefficient (+0.01 to +0.99) (sometimes happens)
         Zero correlation (0.00) (events happen at random)
        Negative correlation coefficient (-0.01 to -0.99) (sometimes)
       Perfect negative correlation coefficient (-1.00) (Never)

    •  Correlations are positive if  the occurrence of the events in each domain increase together.
    •  Correlations are negative if  the occurrence of the events in one domain increases as the occurrence of events in the other domain decreases.

  Correlation is not causation!  it is a clue that can be used to predict.

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Social Psychology
Robert C. Gates