R help - Oct 2010 - summary.lm for post-hoc tests

Let's say I've run Anova(lm(y~a*b)) and found the a:b interaction to be 
significant. Now I'm interested in which specific level combinations of 
a and b significantly differ from the control group. Can I use the 
t-tests from summary(lm(y~a*b)) to answer that question?

I saw no mention of multiple comparison in the documentation for 
summary.lm, so am I right in assuming I need to run p.adjust on the 
p-values column?

Thanks.

PS: I am aware of TukeyHSD and that works fine if a and b are both 
factors. However, if 'a' is numeric and I'm interested in which
levels
of 'b' have a different slope from the control group, TukeyHSD cannot do
that, and I'm hoping the above can.

Check this out:

http://www.rforge.net/doc/packages/NCStats/compSlopes.html

However, it looks like one need to install NCStats
(http://www.rforge.net/NCStats/files/).
Or is there a more "mainstream" method of running multiples
comparisons among slopes?

Dimitri


On Sat, Oct 30, 2010 at 1:20 PM, Alex Bokov <bokov at uthscsa.edu>
wrote:
> Let's say I've run Anova(lm(y~a*b)) and found the a:b interaction
to be
> significant. Now I'm interested in which specific level combinations of
a
> and b significantly differ from the control group. Can I use the t-tests
> from summary(lm(y~a*b)) to answer that question?
>
> I saw no mention of multiple comparison in the documentation for
summary.lm,
> so am I right in assuming I need to run p.adjust on the p-values column?
>
> Thanks.
>
> PS: I am aware of TukeyHSD and that works fine if a and b are both factors.
> However, if 'a' is numeric and I'm interested in which levels
of 'b' have a
> different slope from the control group, TukeyHSD cannot do that, and
I'm
> hoping the above can.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Dimitri Liakhovitski
Ninah Consulting
www.ninah.com