The equation can be rewritten as such :

D = a lg(P/b)

D = a (lg P - lg b)

D = a lg P - a lg b

This is analogous to the equation of a straight line y = mx + c , whereby :

D is the dependent variable. (analogous to y)
a is the gradient of the line. (analogous to m)
lg P is the independent variable. (analogous to x)
-a lg b is the intercept of the vertical axis or the D-intercept in this case. (analogous to c)

(Note that since both a and b are constants,
-a lg b is constant as well)


We will plot D against lg P to obtain a straight line.

Choose the coordinates of any two points on the line. Call them (lg P₁ , D₁) and (lg P₂ , D₂)

Find the ratio of the vertical change over the horizontal change.

a = gradient of line = (D₂ - D₁) / (lg P₂ - lg P₁)

By reading the value of the D-intercept, we can get the value of -a lg b

Eg. The D-intercept = some value k

k = -a lg b
lg b = -k/a

b = 10^(-k/a)