st: Re: what is the difference between '.psel' and '.xbs' for heckman postestimation

[jing liu <pku.liujing@gmail.com>]

> Hi,
>
> I am using heckman regression to calculate the selection effect of wage.
>
> first of all, I am not sure if I shall use the 'twostep' model here or not.
> I sort of know it is related to how to identify the selection effect, but I
> am totally confused with the instruction of 'twostep'.
>     here is the link: http://www.stata.com/help.cgi?heckman
>
> secondly, I want to  interprete the coefficients. So after run 'heckman'
> command, I should use 'predict' command. For stata 8 it was 'predict
> varname. xbs'. The 'xbs' is used to generate the probability that an
> observation will be selected or selection probability. But there is a 'psel'
> subcommand talking about the same thing in Stata11. So now I don't know
> which one i should choose, and what is the difference between the 'xbs' and
> 'psel' now.
>     what stata11 says:
>         xbs: linear prediction for selection equation
>         psel: calculates the probability of selection (or being observed)
>     the link for this is:
> http://www.stata.com/help.cgi?heckman+postestimation
>
> thirdly, it was possible to use the following code to calculate inverse
> mills ratio (gen testpr = normden(selxbpr)/norm(selxbpr)), now we are able
> to use subcommand (mills) to do the same thing, but the command (normden and
> norm) doesn't not exist any more. are they changed? and what are they now?
> forthly,  it says twostep doesn't work with any weight subcommand in heckman
> regression. But I run my heckman with both twostep and pweight/iweight. They
> both work.
>       Stata11 says: pweights, aweights, fweights, and iweights are allowed
> with maximum likelihood estimation; see weight.  No weights are allowed if
> twostep
>       is specified.
> by the way, i have already subscribed my stata11 on line with this email.
> but if you need my serial no. please contact me.
>
> thanks a lot
 --
Best


刘京   Jing LIU

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