At 04:57 PM 4/23/2007, ucb_gal wrote:
>hi, i'm having trouble making a graph from mlogit results. i run my
>mlogit (i'm using multiple imputation, so i use micombine first):
>micombine mlogit depvar black_nh asian_nh native_nh hisp female
>income [pw=custweight], cluster(R234)
>
>next i'm trying to create variables based on the probabilities
>generated from the mlogit:
>prgen income, x(black_nh=1) generate(blk2) ncases(20)
>
>and then i get the error:
>equation 1 not found
>r(303);
>
>what's equation 1? what am i doing wrong?
--- Richard Williams answered:
> There is no guarantee that post-estimation commands are going to work
> correctly after micombine, let alone a user-written post-estimation
> command like prgen.
>
> I don't know if it will help, but I suggest you install -mim- and
> -mimstack-, available from SSC. As Nick Cox posted last month,
> "-mim- replaces Patrick Royston's -micombine- (still available via
> -ssc install ice-) and has in general more facilities and greater
> rigour than -micombine-. It replaces Galati and Carlin's -mitools-.
> " The help for -mim- indicates some things you can and cannot do
> with post-estimation commands. I haven't tried it but it looks like
> mim's -storebv- option might let you do what you want.
Unfortunately, but not surprisingly, -mim- doesn't solve that problem
either. For one thing, you'll probably want to see the probabilities
at the Multiple Imputation estimates of the means of the variables,
and as -prgen- wasn't written with MI in mind it is not surprising
that -prgen- cannot do that. More generally the data is stored in a
way that will confuse any non-multiple imputation program. You'll
have to compute the probabilities yourself, which isn't that hard,
see the example below:
*---------------- begin example ----------------
sysuse auto, clear
recode rep78 1/2 = 3
sum price, meanonly
local min = r(min)
local max = r(max)
tempfile temp
ice rep78 price mpg foreign weight using `temp', m(5)
use `temp', clear
mim: mean weight mpg
/*the means are stored in the matrix e(MIM_Q)*/
matrix mean = e(MIM_Q)
/*the first element is the mean of weight*/
scalar mweight = mean[1,1]
/*and the second element the mean of mpg*/
scalar mmpg = mean[1,2]
mim, storebv: mlogit rep78 price weight mpg foreign
gen xprice = `min' + (_n-1)*(`max' - `min')/19 in 1/20
gen double xb4 = [4]_b[price]*xprice + [4]_b[weight]*mweight /*
*/ + [4]_b[mpg]*mmpg + [4]_b[_cons]
gen double xb5 = [5]_b[price]*xprice + [5]_b[weight]*mweight /*
*/ + [5]_b[mpg]*mmpg + [5]_b[_cons]
gen p3 = 1/(1+exp(xb4)+exp(xb5))
gen p4 = exp(xb4)/(1+exp(xb4)+exp(xb5))
gen p5 = exp(xb5)/(1+exp(xb4)+exp(xb5))
twoway line p3 p4 p5 xprice in 1/20
*----------------- 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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