On 3/23/14 2:47 PM, Tudor Popescu wrote:
Hi Doug,
> 3) In a group X gender X age DODS design, exactly which contrast
> (interaction) has to be examined in order to see if the analysis
> proceeds as DODS or as DOSS? And what would the course of action be
> assuming there are significant clusters for that contrast?
group X gender X age, group X age, gender X age
I've had statisticians tell me that you cannot do anything more if
there
is a sig interaction. However, this kind of thing gets published
all the
time without people even checking. If you wanted to push ahead,
you have
to choose an age where you want to do the test (eg, age=0 which
would be
the default or age=mean age). But you have to be aware that a sig
effect
found betweeen groups will depend on the age that you test it.
Would it not be possible to decompose such a significant interaction
(e.g. group X gender X age) the same way one would decompose any ANOVA
interaction, i.e. by doing a post-hoc contrast to directly compare the
means corresponding to the levels of the factor of interest (e.g.
group A vs group B collapsing across genders) while deciding on an
acceptable value for the factors of non-interest, as you suggest for Age?
Yes, that is fine, you just have to pick an age.
Since such post-hoc comparisons cannot be done directly in QDEC, would
it make sense to instead decompose the interaction by manually
repeating the analysis with only the relevant factors/levels left in
the design?
Technically, that is a different design, and you will not be doing a
post hoc analysis.
doug
Tudor
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