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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: RV: RE: Sum over all possible combinations |

Date |
Mon, 23 Jul 2007 19:11:41 +0100 |

That can happen if you don't tell us the _real_ problem. And you haven't yet, as "a somewhat complex weighting scheme" could mean various things. But yes, Mata would help a lot. You just need to loop doubly over your data. Sorting your data and looping only over non-negative differences and then doubling might help. Nick n.j.cox@durham.ac.uk J E > Thanks to Nick and to Sebastian for their initial responses > to my query I am > afraid, however, that both responses, do not fully solve the > problem I need > to tackle > > In the case of Nick's comment (that the Sum of absolute > differences over all > possible combinations is related to the gini coefficient and > to lmoment > indicators): is true. However since I need to reweight each absolute > difference by a somewhat complex weighting scheme (I did not > posted the > weights to avoid adding complexities to my query), the > shortcut suggested by > Nick would not work in my case. > > The strategy proposed by Sebastian does work: > > bys group: gen g_i=_n > tempfile join_file > preserve > ren y_i y_k > save `join_file', replace > restore > joinby group using `join_file', unmatched(none) > bys group: egen sum_dy = sum(abs(y_i - y_k)) > bys group g_i: keep if _n==1 > > However it only works for relatively small data sets. Very > rapidly the > strategy eats all my memory since it needs to save in memory > a data base > containing all possible combinations. > > Giving this let me ask if somebody will know an alternative > to Sebastian > strategy that will not require this intermediate step. > Alternatively, will > the use of mata will allow me to get around this problem. * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: RV: RE: Sum over all possible combinations***From:*"J E" <je_123@hotmail.com>

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