/* Example 6 - Continuous by Continuous Interactions

*/

clear all
set seed 210317

set obs 60

scalar b0 = 1                  // reference mean | x1==0, x2==0
scalar b1 = 0.75               // x1 slope when x2==0
scalar b2 = 0.75               // x2 slope when x1==0
scalar b12 = -0.75             // change in slope (curvature)
scalar sigma = 0.75            // residual standard deviation

generate x1 = runiform(0,5)
generate x2 = runiform(0,5)
generate mu_y = b0 + b1*x1 + b2*x2 + b12*x1*x2

generate eps = rnormal(0, sigma)
generate y = mu_y + eps

* Several equivalent ways of writing the model specification
regress y c.x1##c.x2
* regress y x1 x2 c.x1#c.x2

* plot 3 slices from predicted surface
margins , at(x1=(0(0.1)5) x2=(0 2.5 5))
marginsplot, title("E(y) ~ x1 | x2") recast(line) recastci(rarea) ciopts(color(*.6))

margins , dydx(x1) at(x2=(0/5))
margins , dydx(x1) at(x2=(0(0.2)2))