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Re: st: reshaping data?
From
Nick Cox <[email protected]>
To
[email protected]
Subject
Re: st: reshaping data?
Date
Fri, 26 Oct 2012 12:51:14 +0100
Sorry, but this seems to imply as many predictors as observations,
which isn't a good idea.
Presumably you don't mean that, so you have tell us more about your
data structure for this to be clear to me.
Nick
On Fri, Oct 26, 2012 at 12:31 PM, <[email protected]> wrote:
> dear Nick,
> I'd like to estimate separate regressions, one for each family, hence the
> number of predictors would be the same for each family
>
> hope this clarifies
> best
> M.
>
>
> Quoting Nick Cox <[email protected]>:
>
>> In that case I really don't understand what you are seeking. See also
>> Yuval's comments.
>>
>> I've never come across a model in which there are a different number
>> of predictors for different observations, although I am always happy
>> to be educated.
>>
>> It seems to me that
>>
>> 1. Either you are applying a standard model, in which case you can
>> give literature references.
>>
>> 2. Or this is a new model, in which case you need to explain how it
>> would be set-up and estimated.
>>
>> Note that it's easy to get variables such as the mean of the other
>> prices in the same family. See
>>
>> FAQ . . Creating variables recording prop. of the other members of a
>> group
>> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J.
>> Cox
>> 4/05 How do I create variables summarizing for each
>> individual properties of the other members of a
>> group?
>> http://www.stata.com/support/faqs/data/members.html
>>
>>
>> On Thu, Oct 25, 2012 at 3:39 PM, <[email protected]> wrote:
>>>
>>> dear Nick,
>>> thanks for your prompt reply. Unfortunately, the number of items is not
>>> fixed
>>> best regards
>>> Maria
>>>
>>>
>>> Quoting Nick Cox <[email protected]>:
>>>
>>>> This model formulation makes me feel a bit queasy, but I think what
>>>> you want is something like this.
>>>>
>>>> Suppose for concreteness the number of items is fixed at 8. (I don't
>>>> see how this will work if the number is not fixed.) So, "for any value
>>>> of 8"
>>>>
>>>> sort code_family code_item
>>>> forval i = 1/8 {
>>>> by code_family : gen P`i' = P[`i']
>>>> }
>>>>
>>>> Note that, contrary to your title, there is no -reshape- here as you
>>>> want your observations to remain observations; at least that's my
>>>> understanding.
>>>>
>>>> Nick
>>>>
>>>> On Thu, Oct 25, 2012 at 3:08 PM, <[email protected]> wrote:
>>>>
>>>>> I have to estimate the equation
>>>>>
>>>>> Q1=a+b1P1+b2P2+...bnPn+e
>>>>>
>>>>>
>>>>> Where Q1 is the quantity of item 1 sold by a supermarket during a week
>>>>> ,
>>>>> P1
>>>>> is the price of item 1 in that week, the other prices are those of the
>>>>> n
>>>>> items belonging to the same family of items. My data set is organized
>>>>> as
>>>>> follows:
>>>>>
>>>>>
>>>>> Code_item week Q P Code_family
>>>>> 789 1 8 2 1
>>>>> 790 1 25 4 1
>>>>> 791 1 9 1.3 1
>>>>>
>>>>> 792 1 12 2 1
>>>>>
>>>>> 800 1 7 2 2
>>>>> 801 1 20 1.2 2
>>>>> 802 1 11 1.6 2
>>>>>
>>>>> 803 1 12 2 2
>>>>>
>>>>> And so forth for the other weeks and families...
>>>>>
>>>>>
>>>>> For each item, how can I include in my regression the prices of the
>>>>> other
>>>>> (n-1) items of the same family, ignoring the prices of the items
>>>>> belonging
>>>>> to other families?
>>
>> *
>>
>> * For searches and help try:
>> * http://www.stata.com/help.cgi?search
>> * http://www.stata.com/support/faqs/resources/statalist-faq/
>> * http://www.ats.ucla.edu/stat/stata/
>>
>
>
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