use  "http://wps.aw.com/wps/media/objects/3254/3332253/datasets2e/datasets/CollegeDistance.dta", clear
reg  ed dist female bytest tuition black hispanic incomehi ownhome dadcoll momcoll cue80 stwmfg80
margins, dydx(dist)

gen ln_ed = ln(ed)
reg  ln_ed dist female bytest tuition black hispanic incomehi ownhome dadcoll momcoll cue80 stwmfg80
predict ln_yb
gen yb=exp(ln_yb)
margins, dydx(dist)

reg  ed dist c.dist#c.dist female bytest tuition black hispanic incomehi ownhome dadcoll momcoll cue80 stwmfg80
margins, dydx(dist) at((means) _all dist=(2 6))

set obs 3807
replace female = 1 in 3797/3807
replace black = 0 in 3797/3807
replace hispanic = 1 in 3797/3807
replace bytest = 58 in 3797/3807
replace dadcoll = 1 in 3797/3807
replace momcoll = 1 in 3797/3807
replace ownhome = 0 in 3797/3807
replace cue80 = 7.1 in 3797/3807
replace incomehi = 0 in 3797/3807
replace stwmfg80 = 10.06 in 3797/3807
replace tuition = .950 in 3797/3807

replace dist = 0 in 3797
replace dist = 1 in 3798
replace dist = 2 in 3799
replace dist = 3 in 3800
replace dist = 4 in 3801
replace dist = 5 in 3802
replace dist = 6 in 3803
replace dist = 7 in 3804
replace dist = 8 in 3805
replace dist = 9 in 3806
replace dist = 10 in 3807

qui reg  ed dist c.dist#c.dist female bytest tuition black hispanic incomehi ownhome dadcoll momcoll cue80 stwmfg80
predict yc

qui reg  ed dist female bytest tuition black hispanic incomehi ownhome dadcoll momcoll cue80 stwmfg80
predict ya

qui twoway (line ya dist in 3798/3807), name(one, replace)
qui twoway (line yc dist in 3798/3807), name(two, replace) 
graph combine one two, name(all, replace)

reg  ed dist c.dist#c.dist female bytest tuition black hispanic incomehi ownhome dadcoll momcoll cue80 stwmfg80 1.dadcoll#1.momcoll
di "Jane vs Mary " 
lincom _b[dadcoll]

di "Alexis vs Mary " 
lincom _b[momcoll]

di "Bonnie vs Mary"
lincom _b[momcoll]+_b[dadcoll]+_b[1.dadcoll#1.momcoll]