Assuming that your initial idea is true (not enough good to comment on it), don't -espoisson- and -ssm- work à la Heckman??? Both available from SSC
Nicola
P.S. I'll NOT receive/read any email but the Digest.
At 02.33 06/06/2009 -0400, John Ataguba wrote:
>Hi Austin,
>
>Specifically, I am not looking at the time dimension of the visits. The data set is such that I have total number of visits to a GP (General Practitioner) in the past one month collected from a national survey of individuals. Given that this is a household survey, there are zero visits for some individuals.
>
>One of my objective is to determine the factors that predict positive utilization of GPs. This is easily implemented using a standard logit/probit model. The other part is the factors that affect the number of visits to a GP. Given that the dependent variable is a count variable, the likely candidates are count regression models. My fear is with how to deal with unobserved heterogeneity and sample selection issues if I limit my analysis to the non-zero visits. If I use the standard two-part or hurdle model, I do not know if this will account for sample selection in the fashion of Heckman procedure.
>
>I think the class of mixture models (fmm) will be an anternative that I want to explore. I don't know much about them but will be happy to have some brighter ideas.
>
>Regards
>
>Jon
>
>
>- ----- Original Message ----
>From: Austin Nichols <[email protected]>
>To: [email protected]
>Sent: Friday, 5 June, 2009 14:27:20
>Subject: Re: st: RE: AW: Sample selection models under zero-truncated negative binomial models
>
>Steven--I like this approach in general, but from the original post,
>it's not clear that data on the timing of first visit or even time at
>risk is on the data--perhaps the poster can clarify? Also, would you
>propose using the predicted hazard in the period of first visit as
>some kind of selection correction? The outcome is visits divided by
>time at risk for subsequent visits in your setup, so represents a
>fractional outcome (constrained to lie between zero and one) in
>theory, though only the zero limit is likely to bind, which makes it
>tricky to implement, I would guess--if you are worried about the
>nonnormal error distribution and the selection b
>
>Ignoring the possibility of detailed data on times of utilization, why
>can't you just run a standard count model on number of visits and use
>that to predict probability of at least one visit? One visit in 10
>years is not that different from no visits in 10 years, yeah? It
>makes no sense to me to predict utilization only for those who have
>positive utilization and worry about selection etc. instead of just
>using the whole sample, including the zeros. I.e. run a -poisson- to
>start with. If you have a lot of zeros, that can just arise from the
>fact that a lot of people have predicted number of visits in the .01
>range and number of visits has to be an integer. Zero inflation or
>overdispersion also can arise often from not having the right
>specification for the explanatory variables... but you can also move
>to another model in the -glm- or -nbreg- family.
>
>On Tue, Jun 2, 2009 at 1:21 PM, <[email protected]> wrote:
>> A potential problem with Jon's original approach is that the use of
>> services is an event with a time dimension--time to first use of
>> services. People might not use services until they need them.
>> Instead of a logit model (my preference also), a survival model for
>> the first part might be appropriate.
>>
>> With later first-use, the time available for later visits is reduced,
>> and number of visits might be associated with the time from first use
>> to the end of observation. Moreover, people with later first-visits
>> (or none) might differ in their degree of need for subsequent visits.
>>
>> To account for unequal follow-up times, I suggest a supplementary
>> analysis in which the outcome for the second part of the hurdle model
>> is not the number of visits, but the rate of visits (per unit time at
>> risk).
>>
>> -Steve.
>>
>> On Tue, Jun 2, 2009 at 12:22 PM, Lachenbruch, Peter
>> <[email protected]> wrote:
>>> This could also be handled by a two-part or hurdle model. The 0 vs. non-zero model is given by a probit or logit (my preference) model. The non-zeros are modeled by the count data or OLS or what have you. The results can be combined since the likelihood separates (the zero values are identifiable - no visits vs number of visits).
>>>
>>>
>>> -----Original Message-----
>>> From: [email protected] [mailto:[email protected]] On Behalf Of Martin Weiss
>>> Sent: Tuesday, June 02, 2009 7:02 AM
>>> To: [email protected]
>>> Subject: st: AW: Sample selection models under zero-truncated negative binomial models
>>>
>>> *************
>>> ssc d cmp
>>> *************
>>> -----Ursprüngliche Nachricht-----
>>> Von: [email protected]
>>> [mailto:[email protected]] Im Auftrag von John Ataguba
>>> Gesendet: Dienstag, 2. Juni 2009 16:00
>>> An: Statalist statalist mailing
>>> Betreff: st: Sample selection models under zero-truncated negative binomial
>>> models
>>>
>>> Dear colleagues,
>>>
>>> I want to enquire if it is possible to perform a ztnb (zero-truncated
>>> negative binomial) model on a dataset that has the zeros observed in a
>>> fashion similar to the heckman sample selection model.
>>>
>>> Specifically, I have a binary variable on use/non use of outpatient health
>>> services and I fitted a standard probit/logit model to observe the factors
>>> that predict the probaility of use.. Subsequently, I want to explain the
>>> factors the influence the amount of visits to the health facililities. Since
>>> this is a count data, I cannot fit the standard Heckman model using the
>>> standard two-part procedure in stata command -heckman-.
>>>
>>> My fear now is that my sample of users will be biased if I fit a ztnb model
>>> on only the users given that i have information on the non-users which I
>>> used to run the initial probit/logit estimation.
>>>
>>> Is it possible to generate the inverse of mills' ratio from the probit model
>>> and include this in the ztnb model? will this be consistent? etc...
>>>
>>> Are there any smarter suggestions? Any reference that has used the similar
>>> sample selection form will be appreciated.
>>>
>>> Regards
>>>
>>> Jon
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