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Re: st: One to N matching


From   "Gao Liu" <gao.liu@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: One to N matching
Date   Tue, 18 Nov 2008 14:11:49 -0500

Thanks, Rich,

Maybe i did not make a clear enough description of my question.

vmatch can match all observations based on certain criteria, like
within some distance. However, what I want is the nearest N
observations, where n is a fixed number. Also, duplicates are not
allowed.

Best

Gao



On Tue, Nov 18, 2008 at 1:24 PM, Richard Goldstein
<richgold@ix.netcom.com> wrote:
> -vmatch- does provide 1 to N because it finds all matches for each case; in
> a recent match that I did I found anywhere from 1 to 18 matches for each of
> my cases
>
> Rich
>
> Gao Liu wrote:
>>
>> Thank you, Richard and Autstin,
>>
>> The score in my dataset is actually from psmatch2. Although psmatch2
>> provides 1 to n matching, but it does not indicate which non-case
>> observations are matched to the case obs, except for the nearest one.
>> Also it allows duplicated matching. That is why I plan to do it by
>> hand.
>>
>> I just checked the command, vmatch. I did not find that it provdes the
>> option of 1 to N matching.
>>
>> Austin, can you decribe some more how to do it by hand.
>>
>> best
>>
>> Gao
>>
>> On Tue, Nov 18, 2008 at 11:32 AM, Austin Nichols
>> <austinnichols@gmail.com> wrote:
>>>
>>> Gao Liu--
>>> Actually, I think you are looking for -psmatch2- (findit psmatch2).
>>> Or did you want to program the matching by hand?  That is also
>>> possible, and not very hard in the case where all you want is the
>>> nearest N matches.  However, note that the order of matching will
>>> matter in the situation you describe--matching without replacement--so
>>> you should probably do the matching many times and compute statistics
>>> using the rules of variance computation for multiple imputation.
>>>
>>> On Tue, Nov 18, 2008 at 11:07 AM, Richard Goldstein
>>> <richgold@ix.netcom.com> wrote:
>>>>
>>>> for already existing programs, rather than writing your own, I would
>>>> start
>>>> with -vmatch- (user-written, type -findit vmatch-)
>>>>
>>>> I'm not sure it will cover your last criterion (used only once) but if
>>>> not
>>>> it should be easy to eliminate those
>>>>
>>>> Rich
>>>>
>>>> Gao Liu wrote:
>>>>>
>>>>> Dear Statlist:
>>>>>
>>>>> I have a question about one to N matching.
>>>>>
>>>>> I have a dataset containing three variables: id, score, case, where
>>>>> case is a dummy variable indicating whether or not the observation is
>>>>> in the case group. How can I match each case observation to N non-case
>>>>> observation based on the score?  Each case observation matches to the
>>>>> N non-case observations with the closest scores, but no case
>>>>> observation can match the same observation (i.e. the non-case
>>>>> observation can be used only one time).
>>>>>
>>>>> Thank you
>>>>>
>>>>> Best
>>>>>
>>>>> Gao
>>>
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