Re: [R] lm - interaction factors - same number of items per level in factors?

From: Ista Zahn <izahn_at_psych.rochester.edu>
Date: Tue, 08 Mar 2011 12:10:32 -0500

Hi,
You should avoid entering interaction terms without also including the lower-order terms involved in the interaction. Also, you are mixing variables from the workspace with those in (the data.frame?) df, which is a very bad idea. Try something like

mod.lm <- lm(res ~ var1+ var2 + var3 + fact1*fact2, data=dat)

Best,
Ista

On Tue, Mar 8, 2011 at 5:19 AM, agent dunham <crosspide_at_hotmail.com> wrote:
> Dear community,
>
> Apologies for my questions, Im pretty newbie.
>
> The thing is, I was trying this lm:
>
> mod.lm <- lm(df$res ~ df$var1+ df$var2 + df$var3 +fact1:fact2)
>
> And factors are like this:
>
>> table(fact1)
> a b c
> 3 90 83
>> table(fact2)
> a b c
> 4 87 85
>
> Because of the interaction, i obtain the following error, (I guess...):
>
>> summary(mod.lm)
>
> Residuals:
> Min 1Q Median 3Q Max
> -3.0163 -1.1604 -0.1741 0.9281 3.8957
>
> Coefficients: (3 not defined because of singularities)
> Estimate Std. Error t value Pr(>|t|)
> (Intercept) 6.5282000 0.7912553 8.250 4.64e-14 ***
> df$var1 0.0005589 0.0003627 1.541 0.1252
> df$var2 -0.2057033 0.0314158 -6.548 6.99e-10 ***
> df$var3 0.0565537 0.0273348 2.069 0.0401 *
> fact1a:fact2a NA NA NA NA
> fact1b:fact2a -0.0086776 1.1375463 -0.008 0.9939
> fact15c:fact2a 0.7215114 1.6043880 0.450 0.6535
> fact1a:fact2b -1.3091987 0.8251966 -1.587 0.1145
> fact1b:fact2b 0.2389941 0.3690889 0.648 0.5182
> fact1c:fact2b 0.5016906 0.3474390 1.444 0.1506
> fact1a:fact2c NA NA NA NA
> fact1b:fact2c 0.3121082 0.3491911 0.894 0.3727
> fact1c:fact2c NA NA NA NA
> ---
> Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1
>
> Residual standard error: 1.562 on 166 degrees of freedom
> Multiple R-squared: 0.3526, Adjusted R-squared: 0.3175
> F-statistic: 10.04 on 9 and 166 DF, p-value: 2.952e-12
>
> Should a, b, and c in the factors have the same number of entries?
>
> Thanks, user_at_host.com
>
>
> --
> View this message in context: http://r.789695.n4.nabble.com/lm-interaction-factors-same-number-of-items-per-level-in-factors-tp3341016p3341016.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help_at_r-project.org mailing list
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org

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Received on Tue 08 Mar 2011 - 17:13:34 GMT

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