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Date:         Tue, 7 Feb 2006 16:35:40 -0300
Reply-To:     Hector Maletta <hmaletta@fibertel.com.ar>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Hector Maletta <hmaletta@fibertel.com.ar>
Subject:      Re: Weight role in using MVA analysis
Comments: To: Rita Clivio <rclivio@tradelab.it>
In-Reply-To:  <LOEFKMFFPMGDNLEBHOFGIEFMCBAA.rclivio@tradelab.it>
Content-Type: text/plain; charset="us-ascii"

You write about weights "less than zero". I assume you mean "less than one". Weights below zero make no sense, and are considered missing by SPSS. If you actually have any negative weights, revise the way they were computed. Weights cannot be negative, and also zero weights mean the case is excluded. Now assuming the weights were not negative, please notice that MVA uses regression for imputation, and for regression and other such procedures weighting is essential to obtain unbiased results from disproportionate samples. Increasing the scale of weights in a uniform manner (e.g. multiplying all of them by 100) would affect the statistical significance SPSS assigns to the results, since probability and standard errors in SPSS are based on total WEIGHTED cases, but otherwise would yield the same results as with your original weights. In your particular case I think significance levels are not particularly important, but beware MVA would use regression estimates it would otherwise consider non significant. Hector

-----Mensaje original----- De: Rita Clivio [mailto:rclivio@tradelab.it] Enviado el: Tuesday, February 07, 2006 3:39 PM Para: Hector Maletta; SPSSX-L@LISTSERV.UGA.EDU Asunto: R: Weight role in using MVA analysis

Thanks Hector I think you're right.

The number of cases that I have weighting the dabse is exactly the number of cases with a weight less than zero.

And so ... do you think it's useful weighting the case before conducting MVA? If so could I use some trick (i.e. weigh * 100) in order to keep the original proportionin dbase ?

Thanks again

Rita

-----Messaggio originale----- Da: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU]Per conto di Hector Maletta Inviato: martedi 7 febbraio 2006 19.15 A: SPSSX-L@LISTSERV.UGA.EDU Oggetto: Re: Weight role in using MVA analysis

One possibility is that some of your weights (40%??) are zero or missing values. Cases with zero or missing weights are not "seen" as cases by SPSS. Zero weights may arise from non-zero fractional weights being rounded down to zero. Hector

-----Mensaje original----- De: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] En nombre de Rita Clivio Enviado el: Tuesday, February 07, 2006 2:56 PM Para: SPSSX-L@LISTSERV.UGA.EDU Asunto: Weight role in using MVA analysis

Hi

I have a question for any kind soul ... :-)

Why if I conduct MVA on a weighted Database I find that I have replaced only the 60% of the cases (i.e. I have a new file with replaced values that is 60% of all cases) ?

If I dont' weight, conducting MVA I have a new file with all values of the dbase.

Weight is assigned to all cases in dbase.

Have you any idea ?

Thank in advance for your help ... and time .

Rita


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