Stata 6: What is the difference between biprobit/heckman and the STB commands?

There are several enhancements to the commands for fitting bivariate probit models (biprobit) in Stata 6.0.

1. One can use weights; fweights, iweights, and pweights are allowed.
2. There are a number of predictions that are allowed:

   p11		Pr(depvar1=1, depvar2=1); the default
   p10		Pr(depvar1=1, depvar2=0)
   p01		Pr(depvar1=0, depvar2=1)
   p00		Pr(depvar1=0, depvar2=0)
   pmarg1		Pr(depvar1=1); marginal probability equation 1
   pmarg2		Pr(depvar2=1); marginal probability equation 2
   pcond1		Pr(depvar1=1 | depvar2=1)
   pcond2		Pr(depvar2=1 | depvar1=1)
   xb1		linear prediction equation 1
   xb2		linear prediction equation 2
   stdp1		standard error of xb1
   stdp2		standard error of xb2

3. The correlation parameter is restricted to [-1,1] interval.
4. The nested model from the STB has been incorporated into the new heckprob command. This new command also offers many predicted values after estimation including

   pmargin		predicted probabilities; Pr(depvar=1); the default
   p11		Pr(depvar=1, selvar=1)
   p10		Pr(depvar=1, selvar=0)
   p01		Pr(depvar=0, selvar=1)
   p00		Pr(depvar=0, selvar=0)
   psel		Pr(selvar=1)
   pcond		Pr(depvar=1 | selvar=1)
   xb		linear prediction
   stdp		standard error of xb
   xbsel		linear prediction in selection equation
   stdpsel		standard error of xbsel