--- Maarten buis <[email protected]> wrote:
> In the example below I simulate the the results if we had the
> continuous data, use -intreg- with log transformed endpoints, and log
> transformed mid-point scaling. All three methods seem to perform ok.
In the example below you see an altarnative simulation in which both
-intreg- and mid-point scaling don't do so well:
*---------------- begin example -------------------
set more off
set seed 12345
sysuse nlsw88, clear
keep collgrad wage
gen lnwage = ln(wage)
reg lnwage collgrad
local pop = _b[collgrad]
capture program drop sim
program sim, rclass
sysuse nlsw88, clear
gen ran = uniform()
sort ran
keep in 1/1000
egen wagecat = cut(wage), ///
at(0, 5, 10, 20, 30, 50)
gen wagecat2 = wagecat
recode wagecat2 ( 0 = 5) ///
( 5 = 10) ///
(10 = 20) ///
(20 = 30) ///
(30 = 50)
gen lncat = ln(wagecat + .5)
gen lncat2 = ln(wagecat2)
intreg lncat lncat2 collgrad
return scalar xcat = _b[collgrad]
gen mid = wagecat
recode mid ( 0 = 3 ) ///
( 5 = 7.5) ///
(10 = 15) ///
(20 = 25) ///
(30 = 40)
gen lnmid = ln(mid)
reg lnmid collgrad
return scalar xmid = _b[collgrad]
gen lnwage = ln(wage)
reg lnwage collgrad
return scalar xcont = _b[collgrad]
end
simulate xcat=r(xcat) xmid=r(xmid) xcont=r(xcont) ///
, reps(1000): sim
twoway kdensity xcont || ///
kdensity xcat || ///
kdensity xmid, ///
xline(`pop') ///
xtitle("effect of collgrad") ///
ytitle("density") ///
legend(order(1 "continuous" ///
"wage" ///
2 "intreg" ///
3 "mid point" ///
"scoring"))
sum
*--------------- end example ------------------
(For more on how to use examples I sent to the Statalist, see
http://home.fsw.vu.nl/m.buis/stata/exampleFAQ.html )
Hope this helps,
Maarten
-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands
visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434
+31 20 5986715
http://home.fsw.vu.nl/m.buis/
-----------------------------------------
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