Hi all,

I have a few questions regarding etregress command, since I intend to incorporate that into a potential project in the future.

To better illustrate, consider this admittedly ridiculous model.

Suppose I wish to regress student test scores on a binary variable that corresponds to whether or not a student smokes.
Etreg allows me to specify another relationship for the binary smoking variable (since the probability of a student can be dependent on his/her environment, income level, etc)

Am I right to specify the equation as follows:
global X " a whole slew of variables that correspond to Individual characteristics"
global Z "characteristics that might be related to someone smoking, but not necessarily related to test scores, which is why these variables only enter in the treatment equation"
global Environment "Environment factors like quality and income level of neighbourhood, etc"

etreg math_score $X, tr(binary_smoking = $X $Environment $Z)

If this is how I ought to specify my model, I then have several follow-up questions.

1) Can I include average test scores as an independent variable in the treatment regression (since I believe probability of smoking is related to how smart you are, potentially) ?

2) Is this test equivalent to a two-stage least squares or simultaneous equation model (reg3 command) ? I ask this because my coefficients for the binary_smoking is well-behaved under etreg (i.e. plausible coefficient values). I get very weird values under "ivregress 2sls" and "reg3" commands, even though I have a semi-decent instrument. Under the etreg command, I get coefficients of a magnitude of 1-2, which is believable since the dependent variable is standardised test scores, but under ivregress and reg3, I get coefficients in the order of -10 and -20, which isn't possible.

3) Lastly, are there any specific econometric/statistics paper that I can be directed to, to further read up on the theoretical underpinnings of this etreg command? The closest thing I've come up with is the Control function approach, popularized by Heckman; but I'm not using the cfunc option (mostly because I can't seem to get it to converge).