Posted by
Maguin, Eugene on
Dec 29, 2014; 5:12pm
URL: http://spssx-discussion.165.s1.nabble.com/REPEATED-MIXED-MODELS-post-hoc-tests-contrasts-tp5728007p5728295.html
Alexandra, I am combining your two emails into one for replying
>>This is from your first email.
The resulting output shows only sign anxiety effect and interaction, and with list length as a fixed factor I obtain an unsignificant result. Which unfortunately undermines the effect of the interaction because without a sign list length effect there isn't an explanation why the interaction is sign (only with harder list lengths anxiety will have a detrimental effect). However only when I add anxiety the effect of list length becomes insign.
This is the output. Should I use a dummy variable for coding the three levels of list length (L1, L2,L3)? how should I interpret an interaction with three levels when LL3 x anx is set to zero?
>> Your results (ignoring gender) are telling you that the relationship between anxiety and lsrt depends on listlength. You should see this if you compute the correlation between anxiety and lsrt within each listlength category. You would also see this if you plotted lsrt against anxiety by listlength and put a regression line in for each listlength category. Those regression lines will not be parallel. Furthermore, I think that the listlength-anxiety correlation is smaller and maybe nearly equal for listlength=1 and 2 because the b coefficients are similar.
>>I strongly urge you to plot lsrt against anxiety by listlength because this plot will give you a visual understanding of what you have. You have a significant interaction between a categorical and a continuous variable. You are looking for what are called regions of significance. Those three regression lines cross (intersect) each other at some values of anxiety--three crosses, three values. From the crossing point for each pair of lines, there are two anxiety values, one to the right of the crossover point and one to the left, at which the difference between the two lines becomes significant. The important thing to understand is that the value at which the difference becomes significant does not have to be a possible anxiety scale score.
>>Finding the significance values is basically algebra but that algebra involves computing the standard error of the difference. Do a google search or a PsychInfo search on "Johnson-Neyman" and "region of significance" (terms "or'ed" not "and'ed"). There's a number of articles but a recent and useful reference is Hayes, Andrew F.; Matthes, Jörg; Behavior Research Methods, Vol 41(3), Aug, 2009. pp.924-936.
>>You can use the emmeans command to identify the boundary points by iteration. The command would be
/EMMEANS=TABLES(LISTLENGTH) with (anxiety=>value<).
>>Don't use a bonferroni adjustment because you are searching for a point rather than conducting a set of tests. As you know you can put a bunch of these emmeans commands in the command syntax so I'd look at the plots and pick a range of values and then iterate in.
>>I think you could also use the Test subcommand; however, I don't directly know how to set that up. I'm sure others do and can offer specifics. I also want to acknowledge that I am unsure of whether there are additional issues or considerations because you are looking at the interaction of a covariate with a repeated factor. Again, I hope that if there are additional issues, a more knowledgeable reader will correct me and give you better advice.
>>This is from your second email.
I have the same question: how can I interpret this interaction? should I use dummy codes for three level within factor interaction (Listlength). Or what does the zero parameter from listlength 3 mean?
>>The zero parameter for listlength 3 means that category was the reference category (this is standard and I've always been curious why that choice was made long ago). Notice that in your first analysis the same category was the reference category. The interpretation is the same as before: the anxiety-lsrt relationship differs across levels of listlength. Dummy codes won't do anything; they will be ignored.
Gene Maguin
-----Original Message-----
From: SPSSX(r) Discussion [mailto:
[hidden email]] On Behalf Of Alexandra
Sent: Monday, December 29, 2014 7:48 AM
To:
[hidden email]
Subject: Re: REPEATED MIXED MODELS-post hoc tests/contrasts
To answer to the other question the database is already in wide format. This is the output for the restructure.
SAVE OUTFILE='C:\Users\Alexandra\Desktop\ls\LSS.sav'
/COMPRESSED.
VARSTOCASES
/MAKE LSRT FROM LS_MEANRT1 LS_MEANRT2 LS_MEANRT3
/INDEX=LISTLENGTH(3)
/KEEP=COD_subiect Trait_anxiety age_group gender
/NULL=KEEP.
Concerning the use of your previous suggested syntax (mixed ls by listlength with anxiety/fixed=listlength anxiety listlength*anxiety /repeated listlength | subject(id) covtype(cs), I changed it a bit in my previous e-mail from today but this would be the output from your syntax (the diff is in the cov type the fact that I used random intercept but in your syntax the repeated command mentioned LISTLENGTH-it's the same thing?)
MIXED LSRT BY LISTLENGTH WITH Trait_anxiety
/CRITERIA=CIN(95) MXITER(100) MXSTEP(10) SCORING(1)
SINGULAR(0.000000000001) HCONVERGE(0,
ABSOLUTE) LCONVERGE(0, ABSOLUTE) PCONVERGE(0.000001, ABSOLUTE)
/FIXED=LISTLENGTH Trait_anxiety LISTLENGTH*Trait_anxiety | SSTYPE(3)
/METHOD=ML
/PRINT=SOLUTION TESTCOV
/REPEATED=LISTLENGTH | SUBJECT(COD_subiect) COVTYPE(CS).
Resulting output (without gender):
Parameter b SE t
Sig.
Intercept 129,3 300,96 ,43 ,66
[LISTLENGTH=1] 455,5 307,3 1,48 ,14
[LISTLENGTH=2] 577,5 307,4 1,87 ,06
[LISTLENGTH=3] 0a 0 .
.
Trait_anxiety 44,28 8,71 5,08 ,000
[LISTLENGTH=1] * Trait_anxiety -27,75 8,89 -3,12 ,002
[LISTLENGTH=2] * Trait_anxiety -25,07 8,90 -2,81 ,005
[LISTLENGTH=3] * Trait_anxiety 0a 0 . .
a. This parameter is set to zero because it is redundant.
b. Dependent Variable: LSRT.
I have the same question: how can I interpret this interaction? should I use dummy codes for three level within factor interaction (Listlength). Or what does the zero parameter from listlength 3 mean?
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