[R] Estimated Effects Not Balanced

Richard M. Heiberger rmh at temple.edu
Mon Aug 22 22:43:39 CEST 2016


The problem is that you have 12 observations and 1+2+10=13 degrees of freedom.
There should be 1 + 2 + 8 = 11 degrees of freedom.
Probably one of your variables is masked by something else in you workspace.
Protect yourself by using a data.frame

> tmp <- data.frame(A=factor(c(1,1,1,1,1,1,2,2,2,2,2,2)),
+ B=factor(c(1,1,2,2,3,3,1,1,2,2,3,3)),
+ y=rnorm(12))
> mod <- aov(y ~ A+B, data=tmp)
> summary(mod)
            Df Sum Sq Mean Sq F value Pr(>F)
A            1  1.553   1.553   1.334  0.281
B            2  3.158   1.579   1.357  0.311
Residuals    8  9.311   1.164

On Mon, Aug 22, 2016 at 11:15 AM, Justin Thong <justinthong93 at gmail.com> wrote:
> Something does not make sense in R. It has to do with the question of
> balance and unbalance.
>
> *A<-factor(c(1,1,1,1,1,1,2,2,2,2,2,2))*
> *B<-factor(c(1,1,2,2,3,3,1,1,2,2,3,3))*
> *y<-rnorm(12)*
> *mod<-aov(y~A+B)*
>
> I was under the impression that the design is balanced ie order does not
> effect the sums of squares. However, when I compute the anova R reports
> that the Estimated Effects are Unbalanced. I thought that when all
> combinations of levels of A and B have equal replications then the design
> is called balanced. But, R tends to think that when not all levels of A and
> levels of B have equal replication, then the "Estimated Effects are
> unbalanced".... Is this the same as the design being unbalanced? Because
> for the example below, where the error occured, the order does not matter
> (which make me think that the design is balanced).
>
>
> *Call:*
> *   aov(formula = y ~ A + B)*
>
> *Terms:*
> *                        A         B Residuals*
> *Sum of Squares   0.872572  0.025604 16.805706*
> *Deg. of Freedom         1         2        10*
>
> *Residual standard error: 1.296368*
> *Estimated effects may be unbalanced*
> --
> Yours sincerely,
> Justin
>
> *I check my email at 9AM and 4PM everyday*
> *If you have an EMERGENCY, contact me at +447938674419(UK) or
> +60125056192(Malaysia)*
>
>         [[alternative HTML version deleted]]
>
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