[R-sig-DCM] Utility scores from mlogit/clogit for CBC

[John Williams]

Hi Jonathan and Chris,

Could you both please join this list?  Every time a non-member posts to 
it I have to manually approve it (which is why your messages are slow to 
appear).

Thanks,

John

On 25/07/13 12:46, Chris Chapman wrote:
> Hi Jonathan --
>
> Yes, in order to estimate the model, it's necessary to use one of the
> levels as the reference attribute. In classic regression terms, it is the
> "intercept" and then the other levels are in reference to that. However,
> when the utilities are reported, they are reported for all levels because
> the utilities are zero-sum.
>
> For instance, suppose that we do regression gives utilities of Brand B =
> 0.5 and Brand C = -0.7 (with Brand A as the reference). Sawtooth, etc.,
> will then report Brand A = 0.2 in order to have sum(utility)==0.  Why?
> Because it is a forced choice, any preference must pick one of the brands,
> and thus p(A | B | C) == 1.0.  To get p(choice)==1.0, it requires
> exp(utilities)==1.0, which requires sum(utilities) = 0 because exp(0)==1.
>   Does that make sense?
>
> As for using R, there are several options and it depends partly on the
> complexity of your analysis and how it was fielded. The package
> "ChoiceModelR" is a fairly easy-to-use package that has much of the
> functionality from bayesm but a simpler interface. There are even easier
> options (such as my own Rcbc code) if you do not require the None option.
> If you need None, then I'd suggest looking first at ChoiceModelR.
>
> I hope that helps,
>
> -- Chris
>
>
> On Thu, Jul 25, 2013 at 3:41 AM, Jonathan Frye <jonathanmfrye at gmail.com>wrote:
>
>> Hello Everyone,
>>
>> I am a graduate student working on a CBC study. I want to give participants
>> 3 choices per task with a None option, so I have been looking at doing a
>> multinomial or conditional logit model. My issue is that in my experience,
>> logistic regressions involve dummy coding or having levels of a factor
>> compare with a base level. So if you have a factor with 3 attributes, the
>> analysis yields 2 coeefficents which give their relation to the third
>> attribute.
>>
>> I thought that the utility scores were equal to the coeeficients, but when
>> I look at other analysis software such as XLstat or Sawtooth, they have a
>> utility score for each attribute.
>>
>> Can anyone explain where I am going wrong? I would greatly appreciate it.
>>
>> I have also considered using the
>> rhierMnlRwMixture<
>> http://127.0.0.1:13720/help/library/bayesm/html/rhierMnlRwMixture.html>
>> command
>> in the bayesm package, but can not figure out if I am setting up the data
>> correctly.
>>
>> Thanks for your time,
>>
>> Jonathan Frye
>> New York University
>>
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>>
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