* Declare panel
xtset firm
* Some lists to be used later
global x84 imum84 fdium84 prod84
global x85 imum85 fdium85 prod85
global x86 imum86 fdium86 prod86
* Bivariate probit
biprobit (ip84 = $x84) (ip85 = $x85) if t==1
* The simultaneous equations bivariate probit is incoherent
biprobit (ip84 = $x84 ip85) (ip85 = $x85 ip84 ) if t==1
* Recursive bivariate probit is an interesting specification
biprobit (ip84 = $x84) (ip85 = $x85 ip84 ) if t==1

* Restrictions - equal coefficient vectors
biprobit (ip84 = $x84) (ip85 = $x85) if t==1
scalar lu = e(ll)
xtprobit ip imum fdium prod if t <= 2, re
scalar lr = e(ll)
display 2*(lu - lr) 

* Recursive bivariate probit model
biprobit (ip84 = $x84 invgood consgood) ///
         (ip85 = $x85 invgood consgood ip84) if t == 1

		 
* An incoherent model
biprobit (ip84 = $x84 invgood consgood ip85) ///
         (ip85 = $x85 invgood consgood ip84) if t == 1
* (This model should be stopped by the software)

* sample selection
heckprob ip84 $x84 if t == 1,select(invgood=prod84 im84)


* sample selection model does not require exclusions
heckprob ip84 prod84 imum84 fdium84 sp84 lsales84 ///
if t == 1,select(invgood=prod84 imum84 fdium84 sp84 lsales84)

* the linear model doesn't either.
heckman ip84 prod84 imum84 fdium84 sp84 lsales84 ///
if t == 1,select(invgood=prod84 imum84 fdium84 sp84 lsales84) two