Monika,
with regard to your first question:
A missing for ll(null) in the output of -lrtest- is nothing to worry
about (and if you look back in your output, they should be missing in
your estimations already). The null likelihood refers to the
likelihood of a model where the only explanatory variable is a
constant. If you did not include a constant term in your models
(option -nocons-), Stata will report a missing value for e(ll_0) in
the regression output, and also in the output of -lrtest-.
This doesn't interfer with the actual likelihood ratio test that
-lrtest- performs (it compares the likelihood of a restricted and an
unrestricted model). Martin already commented on how to interpret the
test result.
Eva
2008/9/16 Monika Baron <[email protected]>:
> Hello everyone!
>
> I'm modelling a Stochastic Cost Frontier with the command frontier.
> I'm trying to find the model's best functional form by using a
> Likelihood-ratio test.
>
> My output:
> ********
> . lrtest A B, stats
>
> Likelihood-ratio test LR chi2(6) = 49.42
> (Assumption: B nested in A) Prob > chi2 = 0.0000
>
> -----------------------------------------------------------------------------
> Model | Obs ll(null) ll(model) df AIC BIC
> -------------+---------------------------------------------------------------
> B | 439 . 548.5923 17 -1063.185 -993.748
> A | 439 . 573.3007 23 -1100.601 -1006.658
> -----------------------------------------------------------------------------
> Note: N=Obs used in calculating BIC; see [R] BIC note
>
> *********
>
> 1. Why are there no ll(null) in the statistics?
>
> 2. What do the results imply? Which model should I choose?
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