RE: [R] Anova - interpretation of the interaction term

From: Ted Harding <Ted.Harding_at_nessie.mcc.ac.uk>
Date: Fri 22 Apr 2005 - 20:52:40 EST


On 22-Apr-05 michael watson \(IAH-C\) wrote:
> [...]
> 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@nessie.mcc.ac.uk> Fax-to-email: +44 (0)870 094 0861
Date: 22-Apr-05                                       Time: 11:52:40
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