What does R mean by "unbalanced design"?

Question

I think this means an unequal sample in different conditions. But it seems to mean something else. . .

I have a data set like below

particip    group   device  width   length  accep   thresh  rating  d-rating
1           RA      Dingo   nom     nom     Y       5       8       3
1           RA      Dingo   nom     long    Y       4       6       2
1           RA      Dingo   fat     nom     Y       4       6       2
1           RA      Dingo   fat     long    N       6       4      -2

and I'm running an ANOVA on it like so

aov.AMIDS_d <- aov(d.rating ~ group*device*width*length + Error(particip/(device*width*length))+group,data.AMIDS_d) 

This works ok until I try to print the condition means like so

print(model.tables(aov.AMIDS_d,"means"),digits=3)

and it says

Error in model.tables.aovlist(aov.AMIDS_d, "means") : design is unbalanced so cannot proceed

According to the design, it ought to be balanced, so I need to check my data structure. I tried

table(data.AMIDS_d[,2:5])

to give a table of observations per condition and got this

, , width = fat, length = long

     device
group Dingo SNAR
   NR    12   12
   NV    12   12
   RA    12   12

, , width = nom, length = long

     device
group Dingo SNAR
   NR    12   12
   NV    12   12
   RA    12   12

, , width = fat, length = nom

     device
group Dingo SNAR
   NR    12   12
   NV    12   12
   RA    12   12

, , width = nom, length = nom

     device
group Dingo SNAR
   NR    12   12
   NV    12   12
   RA    12   12

which looks both correct and balanced. So what is causing the unbalanced design error?

Answer 1

In case anyone else has this problem, check that your all your factor columns, particularly any containing numbers and including your participant-identifier column, are classed as a Factor and not as an Integer by using str(yourdata) or class(yourdata$columnname). My particular culprit was the participants column.

If it's classed as an Integer, then

yourdata$columnname <- as.factor(yourdata$columnname)

will re-class it as a factor.