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From |
"Nick Cox" <[email protected]> |

To |
<[email protected]> |

Subject |
RE: st: heckman sample selection ML |

Date |
Tue, 13 Dec 2005 22:15:41 -0000 |

log (y + 1) is not quite so ad hoc as this implies. If values are zero or positive counts, then ln(y + 1) is 0 up. If values are positive counts, then ln y is 0 up. Constants between 0 and 1 inclusive won't do this for you. On the whole, however, I agree with the thrust of Robert's argument that there are better ways of coping with zeros. Nick [email protected] Robert Duval > Which raises then the issue... why adding "+1" to y?... just to be > able to take logs? > then why not adding 3/4 or 1/2 or 1/4 for that matter? > > The problem with this solution is that, by picking an arbitrary number > to add, you alter the curvature on your transformed dependent variable > (y+1) in a pretty ad hoc manner. > > A more sensible solution woud be to do the log transformation on y > whenever y>0, and deal with the zeroes (now missing) directly in your > econometric method, be it Hecman SS correction, Tobit, or whatever way > you want to deal with censoring. > On 12/13/05, Maarten buis <[email protected]> wrote: > > at Tue 12/13/2005 1:59 PM Victoria Levin wrote: > > > My original dependent variable was left-censored at zero, but > > > since I had to take logs, I did a monotonic transformation of > > > the dependent variable (y+1), and then took logs of that > > > [ln(y+1)]. So, my current dependent variable has many zeroes; > > > the rest are positive values. > > > > > > 1) Somehow, when I run the full MLE heckman model: > > > > > > heckman y x1 x2 x3, select (x1 x2 x3) > > > > <snip> > > > > > I get the following error: > > > > > > Dependent variable never censored due to selection: > > > model would simplify to OLS regression > > > > -heckman- assumes that an observation is censored if y has > a missing value. You ensured that there > > are no missing values, but by doing so prevented -heckman- > from identifying the censored > > observations. So just taking the log of y would be enough > in your case. * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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