[R] Asking, are simple effects different from 0

[jebyrnes]

Ah.  I see.  So, if I want to test to see whether each simple effect is
different from 0, I would do something like the following:


cm2 <- rbind( 
"A:L" =  c(1, 0, 0, 0, 0, 0), 
"A:M" = c(1, 1, 0, 0, 0, 0), 
"A:H" = c(1, 0, 1, 0, 0, 0), 
"B:L" =   c(1, 0, 0, 1, 0, 0), 
"B:M" = c(1, 1, 0, 1, 1, 0), 
"B:H" = c(1, 0, 1, 1, 0, 1))

summary(glht(fm, linfct = cm2), test = adjusted(type="none")) 

Fit: lm(formula = breaks ~ tension * wool, data = warpbreaks)

Linear Hypotheses:
         Estimate Std. Error t value  p value    
A:L == 0   44.556      3.647  12.218 2.22e-16 ***
A:M == 0   24.000      3.647   6.581 3.23e-08 ***
A:H == 0   24.556      3.647   6.734 1.88e-08 ***
B:L == 0   28.222      3.647   7.739 5.47e-10 ***
B:M == 0   28.778      3.647   7.891 3.22e-10 ***
B:H == 0   18.778      3.647   5.149 4.84e-06 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 
(Adjusted p values reported -- none method)

Correct? What is the df on those t-tests then?  Is it 48?

Interestingly, I find this produces results no different than

fm2<-lm(breaks ~ tension:wool+0, data=warpbreaks) 
summary(fm2)

Also, here, it would seem each t-test was done with the full 48df.  Hrm.


Chuck Cleland wrote:
> 
> 
>    Each column corresponds to one of the coefficients in the model, and 
> each row specifies a particular contrast.  The numbers in the matrix 
> indicate how the model coefficients are combined to indicate a 
> particular difference in means.
>    For example, the first row indicates that the third coefficient 
> (woolB) is multiplied by -1.  The baseline categories are A and L for 
> the wool and tension factors, so the woolB effect in fm is the simple 
> effect of B vs. A in the baseline category of the tension factor. 
> Multiplying this coefficient by -1 produces an A vs. B comparison in the 
> baseline category of the tension factor.
>  
> 
> 

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