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

[agent dunham]

Dear community, 

Apologies for my questions, I´m 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


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