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Re: st: Estimating Logit Price Response Function With Unknown Market Size


From   Nick Cox <njcoxstata@gmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Estimating Logit Price Response Function With Unknown Market Size
Date   Tue, 3 Dec 2013 17:40:25 +0000

What's holding you back? You can try it yourself and see.

input Price   Cumulative
<your data>

nl log3 C P
predict predicted
twoway connected C P || line predicted P, sort

Answer: not too bad, gets it wrong for high prices.

Nick
njcoxstata@gmail.com


On 3 December 2013 17:27, Marigliano, Joseph
<JMarigliano@premierfarnell.com> wrote:
> Nick, thanks. I have taken a look.
> Here is the example data:
>
> Price   Cummulative Quantity
> 0.1     8100
> 0.2     8000
> 0.3     7800
> 0.4     7500
> 0.5     6700
> 0.6     5700
> 0.7     4000
> 0.8     2500
> 0.9     1200
> 1       700
> 1.1     500
> 1.2     400
> 1.3     350
> 1.4     300
> 1.5     290
>
> I am trying to fit this as a Reverse Sigmoid curve. Will nl log3 still
> be appropriate in this case?
>
> Regards,
>
> Joseph
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> Sent: Wednesday, November 27, 2013 1:12 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Estimating Logit Price Response Function With Unknown
> Market Size
>
> You asked the same question on 22 November.
>
> When there is no response, often a question is perceived as not being
> clear enough to answer.
>
> I read this as fitting a logistic growth curve to time series. The upper
> asymptote is unknown and thus a parameter to be estimated. This is a
> classic for -nl-: indeed a sample program -log3- for precisely this
> model is bundled with Stata. Have look at the manual entry for -nl-.
>
> Nick
> njcoxstata@gmail.com
>
>
> On 27 November 2013 15:47, Marigliano, Joseph
> <JMarigliano@premierfarnell.com> wrote:
>
>> I have a file containing historical sales data by product.
>> It contains product and cumulative sales by price point for the last 6
>
>> months.
>> I am looking to estimate the curve as a logit, but do not know the
>> market size.
>> Are there any good examples of this?
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