On the  Basis  of Ratio of Change  Direction

 a.Linear Correlation: The  correlation  between  two variables a said  to be liner if  corresponding  to a unit change in the values  of one  variable there is  a constant  change in the  value of the  other variable  i ,e,  in case  of linear  correlation  the  relation  between  the variables X and Y is  of the type.

Y= a + b x

If a = 0  the relation  becomes  y =  bx. In such cases  the values  of the variables are in constant  ratio.

Non linear ( Curvi linear) Correlation: The correlation  between  two variables  is said to be  non linear  or curvilinear  in  corresponding  to a unit  change in the value of one variable   the  other  variable does  not changes  at a constant  rate but at a fluctuating rate.