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From |
Nick Cox <njcoxstata@gmail.com> |

To |
agostino@unical.it |

Subject |
Re: st: reshaping data? |

Date |
Thu, 25 Oct 2012 15:48:16 +0100 |

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, <agostino@unical.it> wrote: > dear Nick, > thanks for your prompt reply. Unfortunately, the number of items is not > fixed > best regards > Maria > > > Quoting Nick Cox <njcoxstata@gmail.com>: > >> 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, <agostino@unical.it> 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/

**Follow-Ups**:**Re: st: reshaping data?***From:*agostino@unical.it

**References**:**st: reshaping data?***From:*agostino@unical.it

**Re: st: reshaping data?***From:*Nick Cox <njcoxstata@gmail.com>

**Re: st: reshaping data?***From:*agostino@unical.it

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