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From |
"Brian P. Poi" <bpoi@stata.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Bootstrap in two-stage models |

Date |
Fri, 18 Feb 2005 10:28:11 -0600 (CST) |

On Fri, 18 Feb 2005, nolwenn roudaut wrote:

Dear All,

I want to estimate a treatment effect model, a two-stage model with a probit

model in the first and an OLS in the second.

I cannot use the 'treatreg' stata program as some of my hypothesis are different

from the classical ones.

So I need to compute the two-step corrected variance-covariance matrix "by

hand".

One way to do this is to use the Murphy-Topel correction procedure. (I thank a

statalister that provided me his program last month).

Another way (and this is the one I am interested in) is to use bootstrap

technics.

Does anyone knows how to implement this method?

Nolwenn

One alternative may be to use the -bootstrap- command along with -treatreg, twostep-. That would provide bootstrap standard errors for the classic two-step treatment effects model. In the program below I do just that, using the dataset described in [R] treatreg. Then I write my own program that illustrates how to replicate the results from -bootstrap- and -treatreg, twostep-. The only thing of note is that even though we are bootstrapping, to use -ereturn post- we must provide a variance matrix; the identity matrix is fine as long as the row and column names match the column names of our parameter vector. webuse labor, clear gen wc = cond(we>12, 1, 0) set seed 1 bootstrap "treatreg ww wa cit, treat(wc=wmed wfed) twostep" _b, reps(50) program mytreat, eclass // Stage 1 probit probit wc wmed wfed tempname b mat `b' = e(b) // Get the hazard ratio tempvar xb hazard predict double `xb', xb generate double `hazard' = . replace `hazard' = normden(`xb') / norm(`xb') if wc==1 replace `hazard' = -1*normden(`xb') / (1-norm(`xb')) if wc==0 // Now do the second step regress ww wa cit wc `hazard' tempvar used gen byte `used' = e(sample) mat `b' = `b', e(b) // `b' has [<probit coefs> <reg coefs>] mat colnames `b' = pr_wmed pr_wfed pr_cons wa cit wc hazard _cons // Need a variance matrix for -ereturn post- // The identity matrix is fine, since we don't // really care what is in it. tempname V mat `V' = I(colsof(`b')) mat rownames `V' = pr_wmed pr_wfed pr_cons wa cit wc hazard _cons mat colnames `V' = pr_wmed pr_wfed pr_cons wa cit wc hazard _cons ereturn post `b' `V', esample(`used') end set seed 1 bootstrap "mytreat" _b, reps(50) Hope this helps -- Brian Poi -- bpoi@stata.com * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Bootstrap in two-stage models***From:*nolwenn roudaut <nolwenn.roudaut@univ-tlse1.fr>

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