st: interactions, slopes, and standard errors

[Chip Hunter <Lhunter@bus.wisc.edu>]

Has anyone written a general program to calculate and display the estimated 
slope and associated error of Y on x1 at various values of x2, where Y = b0 
+ b1x1 + b2x2 + b3x1x2

Last fall Scott Merryman posted a nice program (which I copy in, below) 
which takes care of this at the mean of x2. Thanks, Scott!, We could also 
be interested in doing it for other values.

Chip Hunter

**
- Scott Merryman's program to calculate the marginal effects and the 
variance of interaction terms -

program marginalx, rclass
version 8.1
syntax , x(varname) y(varname) xy(varname)

sum `y' if e(sample), meanonly

local marginal = _b[`x'] + _b[`xy']*r(mean)

matrix A = e(V)
matrix B1 = A["`x'" , "`x'"]
local var_b1 = B1[1,1]

matrix B2 = A["`xy'" , "`xy'"]
local var_b2 = B2[1,1]

matrix B3 = A["`x'" , "`xy'"]
local covar_b1b2 = B3[1,1]

local variance = `var_b1' + (r(mean)^2)*`var_b2' + 2*r(mean)*`covar_b1b2'

dis ""
disp in gr "Marginal effect of `x' at the mean of `y' = " `marginal'
disp in gr "Variance of marginal effect of `x' at the mean of `y' = " 
`variance'

return scalar margx = `marginal'
return scalar var_margx = `variance'
end




**************************************************

Chip Hunter
University of Wisconsin-Madison
School of Business
Management and Human Resources Dept.
4114 Grainger Hall
975 University Ave.
Madison, WI 53706
608-265-3533
**************************************************

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