Using nonlinear regression to compare slopes and intercepts of linear regression lines.

Prism offers a choice in linear regression to compare slopes and intercepts. If you use nonlinear regression to fit a line, you get a bit more flexibility. Use the Constrain tab to specify which parameter is shared (or share both). Use the Compare tab to set up the comparison.

Using different criteria for pooling the slope

With linear regression, Prism pools he slope if the P value for comparing slopes is greater than 0.05. You may want to use a different cutoff. With nonlinear regression you can choose which parameters to pool or not. 

Forcing the line through the origin

The feature built in to linear regression will not work if you force the lines to go through the origin. But Prism  can do the comparison. The trick is not to use "nonlinear" regression instead, and choose the straight line equation ("first order polynomial"). Constrain the intercept to zero and choose to compare slopes (in the compare tab of nonlin regression).

This file demonstrates how to compare slopes and intercepts with nonlinear regression.