Re: zero times sysmis equals 0. why?

Posted by Bruce Weaver on Mar 16, 2011; 3:49pm
URL: http://spssx-discussion.165.s1.nabble.com/zero-times-sysmis-equals-0-why-tp3783714p3785778.html

I'll call your nominal variable FEMALE and your scaled variable X.  If you have FEMALE*X in the model, then you should have both FEMALE and X in the model too.  So any cases where X is missing will be excluded from the model, regardless of whether FEMALE*X is valid or missing.

HTH.

Alison Papadakis wrote

Martin posted the question for me.  I just subscribed to the list.

This default was problematic because I was using a dummy coded nominal variable (males = 0, females = 1) multiplied by a scale variable to create an interaction term.  In that case, the default mathematical assumption of anything times 0 is 0 does not hold.  A missing value times a dummy coded value of 0 should return a missing.

Most SPSS documentation I have seen suggests any operation or function incorporating a missing value results in a missing value, which made the result surprising.  Since the testing of moderation using the product of a dummy coded variable is a common approach, I am concerned about how many people may make the same error that I did and not realize it.  Of course an if statement added to the computation of the interaction term works as a workaround for those who are aware of the default, but how many are aware of the default?

From: SPSSX(r) Discussion [mailto:[e-mail]] On Behalf Of Jon K Peck
Sent: Wednesday, March 16, 2011 11:12 AM
To: [e-mail]
Subject: Re: zero times sysmis equals 0. why?

It has been like that forever.  You can find a table of these results in the CSR.

Whether this is appropriate depends on how you view SYSMIS.  Since x/0 is SYSMIS, you could argue that 0*SYSMIS should be SYSMIS, but if SYSMIS results from something finite, then 0*SYSMIS should be zero.

Regards,

Jon Peck
Senior Software Engineer, IBM
[e-mail]
312-651-3435



From:        Martin Sherman <[e-mail]>
To:        [e-mail]
Date:        03/16/2011 08:51 AM
Subject:        [SPSSX-L] zero times sysmis equals 0. why?
Sent by:        "SPSSX(r) Discussion" <[e-mail]>
________________________________



Dear list: I just multiplied zero times sysmis and the result was a zero.  I wasn’t expecting that.  Is that something new to IBM SPSS 19.0?

Martin F. Sherman, Ph.D.
Professor of Psychology
Director of  Masters Education in Psychology: Thesis Track

Loyola University Maryland
Department of Psychology
222 B Beatty Hall
4501 North Charles Street
Baltimore, MD 21210

410-617-2417
[e-mail]<mailto:[e-mail]>