If you use the listwise option in your regression analysis, multiplying by a dummy-coded 0 wont cause a problem because cases with a missing value on any variable in the regression will be automatically excluded. Fortunately, listwise is the SPSS default.
Garry Gelade
Business Analytic Ltd
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of Alison Papadakis
Sent: 16 March 2011 15:39
To: [hidden email]
Subject: Re: zero times sysmis equals 0. why?
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:[hidden email]] On Behalf Of Jon K Peck
Sent: Wednesday, March 16, 2011 11:12 AM
To: [hidden email]
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
[hidden email]
312-651-3435
From: Martin Sherman <[hidden email]>
To: [hidden email]
Date: 03/16/2011 08:51 AM
Subject: [SPSSX-L] zero times sysmis equals 0. why?
Sent by: "SPSSX(r) Discussion" <[hidden email]>
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
[hidden email]
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