# "Estimated effects may be unbalanced" message when running aov in R. What does it mean?

I am running a summary(aov(...)) in R and I got this message:

Estimated effects may be unbalanced


What does it mean? How may I solve this problem?

aov is designed for balanced data (link). Balanced design is: An experimental design where all cells (i.e. treatment combinations) have the same number of observations (link).
• I actually have faced the this is not necessarily true(Correct me if I am wrong). If you have an experiment with two factors. 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) aov(y~A+B) gives the error prompt. We can see that all combination of levels A and B have equal replications , yet the "Estimated Effects may be unbalanced". I have a feeling that R compares the replication of levels of A with replications of levels of B. Aug 22, 2016 at 14:46
Install package called car & activate it first and then calculate sum of squares using Anova(lm(y~x1*x2),type=2)