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Re: st: 2SLAD - qreg - how to obtain t-stat and p-value


From   "Brian P. Poi" <bpoi@stata.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: 2SLAD - qreg - how to obtain t-stat and p-value
Date   Wed, 7 Apr 2004 15:41:50 -0500 (CDT)

Earlier today (Apr 7, 2004) Thi Minh asked

> I am now estimating a 2SLAD model, and follow the instructions given by
> Statacorp's Brian P. Poi in the statalist dated 26 Sep 2003.  Is there a
> way to retrieve the p-value and the t-stat as they appear when using the
> command bsqreg? As Brian Poi explains, the s.e. need to be bootstrapped
> for _both_ stages of the estimation so that it is the command
> _bootstrap_ which needs to be used.

> program bootit2
>       version 8.0
>       //region 2 - stage 1
>       reg Y2 X1 Z if reg7==2
>       predict double phat2, xb
>
>       //region 2 - stage 2
>       qreg Y1 X1 phat2 if reg7==2
>       drop phat2
> end
> bootstrap "bootit2" _b, reps (200) dots


In [R] -qreg- we give the following example of -bsqreg-:

        sysuse auto, clear
        set seed 1001
        bsqreg price weight length foreign , reps(1000)

We can replicate the output table including t-statistics, p-values
and confidence intervals ourselves using -bootstrap- and -qreg-.
This code does just that:

        set seed 1001
        bootstrap "qreg price weight length foreign" _b, reps(1000)
        // The following code should work after any call to
        // -bootstrap-
        matrix b = e(b)
        matrix se = e(se)
        local names : colnames(se)
        local df = e(N) - colsof(b)
        tempname t lb ub
        local i = 1
        foreach var of local names {
                scalar `t' = (b[1, `i'] / se[1, `i'])
                scalar `lb' = b[1, `i'] - se[1, `i']*invttail(`df', 0.025)
                scalar `ub' = b[1, `i'] + se[1, `i']*invttail(`df', 0.025)
                di "`var'" _col(15) %9.0g b[1,`i'] ///
                           _col(26) %9.0g se[1, `i'] ///
                           _col(38) %6.2f `t' ///
                           _col(46) %6.3f 2*ttail(`df', abs(`t')) ///
                           _col(56) %9.0g `lb' ///
                           _col(68) %9.0g `ub'
                local i = `i' + 1
        }



These results agree with -bsqreg-, since all we've really done is
replicate what -bsqreg- does automatically.

The same code that appears after my call to -bootstrap- can be used
any bootstrapping job, including bootstrapping the 2SLAD estimator
that Thi Minh mentioned.

Hope this helps

        -- Brian Poi   
        -- bpoi@stata.com


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