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Association.
Two variables are associated if variation in one variable has effect on variation in other variables.

Correlation.
It is a measure of association between variables.

Scatter Diagram or Scatter Plot
It is graph obtained by plotting of values of two variables which describe single bivariate observation (e.g. height and weight of a person). One variable (independent variable) X coordinate and the other variable (dependent variable) as Y coordinate.

Correlation coefficient.
The correlation coefficient r is a measure of how nearly a scatter diagram or scatter plot falls on a straight line. The correlation coefficient is always between -1 and +1.

Causation, causal relation.
Two variables are causally related if changes in the value of one cause the other to change.

END OF CHAPTER ONE.