Correlation coefficient (r)

Correlation coefficient measures the degree of association between two variables such as the cost and the activity level.

r = nΣxy   -  Σx  Σy
     [nΣx² – (Σx)² ] [nΣy²  - (Σy)²]

When the degree of association among the two variables is very close then it would be almost possible to plot the observation on a straight line and r will be almost equal to one.

-1≤ r ≤ 1

If r= -1, then the two variables are said to be perfectly negatively correlated
If r = +1, then the two variables are perfectly positively correlated
If r = 0, then there is no correlation between the two variables.