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From: | Alan Mead |
Subject: | Re: PSPP linear regression (reference category). |
Date: | Thu, 29 Sep 2016 10:17:51 -0500 |
User-agent: | Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 |
I've always used syntax to create dummy variables and it works the
same in PSPP and SPSS. If you had a categorical variable X with three values 0/1/2 you would create two dummy coded variables like this: recode x (0=1) (1=0) (2=0) (ELSE=SYSMIS) into x_dum1. recode x (0=0) (1=1) (2=0) (ELSE=SYSMIS) into x_dum2. execute. This creates two dummy variables called x_dum1 and x_dum2 and they encode the effect for category 0 and 1 (respectively) vs. category 2 as a reference. If you wanted to use, say, group 1 as the reference, then you make sure that 1 is encoded as zero for both dummy variables: recode x (0=1) (1=0) (2=0) (ELSE=SYSMIS) into x_dum1. recode x (0=0) (1=0) (2=1) (ELSE=SYSMIS) into x_dum2. execute. Sometimes to give myself more flexibility, I create dummy codes for all three values and only pick two in any analysis. This forces the category of the left out dummy variable to be the reference: recode x (0=1) (1=0) (2=0) (ELSE=SYSMIS) into x_dum0. recode x (0=0) (1=1) (2=0) (ELSE=SYSMIS) into x_dum1. recode x (0=0) (1=0) (2=1) (ELSE=SYSMIS) into x_dum2. execute. For example, if you enter x_dum0 and x_dum2 then category 1 is the reference. If you forget and enter all three dummy variables the regression should fail because the data matrix lacks full rank. -Alan On 9/29/2016 9:58 AM, Jack Drew wrote:
-- Alan D. Mead, Ph.D. President, Talent Algorithms Inc. science + technology = better workers +815.588.3846 (Office) +267.334.4143 (Mobile) http://www.alanmead.org I've... seen things you people wouldn't believe... functions on fire in a copy of Orion. I watched C-Sharp glitter in the dark near a programmable gate. All those moments will be lost in time, like Ruby... on... Rails... Time for Pi. --"The Register" user Alister, applying the famous "Blade Runner" speech to software development |
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