Anova - interpretation of the interaction term

[...]
So from a statistical point of view, I can see exactly why the
interaction term is significant, but what my colleage wants to know is
that WITHIN the vaccinated group, does infection decrease expression
significantly?  And within the unvaccinated group, does infection
decrease expression significantly?  Etc etc etc  Can I get this
information from the output of the ANOVA, or do I carry out a separate
test on e.g. just the vaccinated group? (seems a cop out to me)

If I understand right, each of these questions can only be answered
in terms of the changes in mean level *within* group.

However, you are entitled to use the residual sum of squares
(after estimating both effects and interaction) for the estimate
of SE to which you compare these within-group changes. Provided,
of course, that the variance is homogenous across groups (i.e.
treatment and/or infection has no influence on variability).

You can get the latter from the original ANOVA (interaction term
included) but I think you should get the difference of means
(infected vs non-infected) by a separate anlysis of each group.

Many thanks, and sorry, but it's Friday.

Don't apologise. Not your fault it's Friday. (Who *can* I blame?)

Cheers,
Ted.


--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 22-Apr-05                                       Time: 11:52:40
------------------------------ XFMail ------------------------------