[R] Asking, are simple effects different from 0

[jebyrnes]

Ah, I see where we are talking past each other.  In my particular analysis
(I'm looking at deviations from a predicted value), any deviation from 0
(whether due to grand mean or not) is actually very very interesting.  What
is ultimately interesting to me is the sign of that difference, but, I need
to establish any difference at all first.  The model I should actually be
working with is more like lm(response ~ trta*trtb+0), really.  But, still,
in evaluating the glht output, is it fair to use the 48 df, or should I use
the df for each cell?  I think that's where I've been getting hung up.


Chuck Cleland wrote:
> 
> On 3/5/2008 3:19 PM, jebyrnes wrote:
>> Indeed, but are not each of the cell means also evaluations of the effect
>> of
>> one factor at the specific level of another factor?  Is this an issue of
>> "Tomato, tomahto".
> 
>    I don't think it is "tomato, tomahto".  Say the grand mean is around 
> 100 and the within cell standard deviations are around 10.  You could 
> easily have a situation in which all of the cell means are significantly 
> different from 0, but there is nothing at all interesting going on with 
> the two explanatory factors.  In other words, the cell means can be very 
> different from 0 with no explanatory variable effects of any kind, based 
> only on the overall location of the response.
> 
>> I guess my question is, if I want to know if each of those is different
>> from
>> 0, then should I use the 48df from the full model, or the 9 for each
>> cell?
> 
>> Chuck Cleland wrote:
>>>    That does not corresponds to what I think of as the simple effects. 
>>> That specifies the six cell means, but it does not *compare* any cell 
>>> means.  I think of a simple effect as the effect of one factor at a 
>>> specific level of some other factor.
>>>
>>>> summary(glht(fm, linfct = cm2), test = adjusted(type="none")) 
>>>>
>>>> Correct? What is the df on those t-tests then?  Is it 48?
>>>    Yes, df = 48 for each contrast.
>>>
>>>> Interestingly, I find this produces results no different than
>>>>
>>>> fm2<-lm(breaks ~ tension:wool+0, data=warpbreaks) 
>>>> summary(fm2)
>>>    Yes, but those are not what I would call the simple effects.  Those 
>>> are essentially one-sample t-tests for each of the 6 cell means.
>>>
>>>> Also, here, it would seem each t-test was done with the full 48df. 
>>>> Hrm.
>>>    The df are based on the whole model, not the 9 observations in one
>>> cell. 
> 
> -- 
> Chuck Cleland, Ph.D.
> NDRI, Inc.
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