Date: Thu, 29 Sep 2005 18:02:51 0300
ReplyTo: Hector Maletta <hmaletta@fibertel.com.ar>
Sender: "SPSSX(r) Discussion" <SPSSXL@LISTSERV.UGA.EDU>
From: Hector Maletta <hmaletta@fibertel.com.ar>
Subject: Re: Weighted Correlation
InReplyTo: <5101F2BA382BB245A7033EC1E5B8E35501C7B4B7@namail2.na.ipsos>
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1. You are not doing right. In my opinion your procedure makes no sense.
Correlation should not get greater or smaller depending on the number of
cases. What may increase with the number of cases is their statistical
significance: a coefficient of 0.30 may not be significant with a few cases,
but significant with more cases. However, even this would be improper in
your cases, because for variables other than X1 you did actually have fewer
cases, and just telling the computer to pretend it has a larger number of
cases would not increase the intrinsic significance of your correlations,
though it will make them APPEAR to be more significant to uncautious
readers.
2. There MIGHT be a way out if you could ESTIMATE the value of other
variables (other than X1) based on information actually given by those
subjects. The SPSS MISSING VALUES module (and other softwares) allows you to
estimate the expected value of, say, X2 for a subject lacking that
information, based on a regression of X2 on other variables where that
subject gave valid responses. Estimating is one thing and observing is
another. However, these estimated or imputed values for the other variables
would give you some indication of the correlation of X2 with X1 including
all cases for whom the estimation can be carried out. Advice: if these
estimated cases are many, abstain. Do it only if they are few, and only if
the regression estimates have a small margin of error (i.e. very high
significance).
3. Your subject line reads "Weighted correlation", but that's a misnomer:
what you proposed to do is not weighting, but rigging the correlation to
make them appear larger.
Hector
> Original Message
> From: SPSSX(r) Discussion [mailto:SPSSXL@LISTSERV.UGA.EDU]
> On Behalf Of Siraj Urrehman
> Sent: Thursday, September 29, 2005 5:46 PM
> To: SPSSXL@LISTSERV.UGA.EDU
> Subject: Weighted Correlation
>
> Hi Guys
> I have ten variables and except the first variable (X1) all
> have so many missing observations. I calculated the
> correlations between X1 and rest of the nine variables. So I
> have nine correlation values with nine different bases
> depending on X2 to X10. I want to calculate the correlation
> values with the base of X1. I tried to do as
>
> (Correlation of X1 and X2)*(# of non missing values in X1 AND
> X2)/(# of values in X1) (Correlation of X1 and X3)*(# of non
> missing values in X1 AND X3)/(# of values in X1)
>
> And so on. My question am I doing right or there is another
> way? Thanks Siraj
>
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