It's difficult to give definitive advice about what it means, but the
simplest approach is to look at what it means: plot the model and see
what it looks like. Basically, the interaction says that as the
variables both increase, the relationship gets less negative. The
details depend on the ranges of the variables, hence plotting the
results is helpful.
I would write some code, but that would probably mainly show that I'm
out of date with R. But you could use expand.grid() to create new data
to predict, use predict() to predict it, and then plot it as several
lines on one plot (e.g. plot against C.abundances, with one line for
each value of C.diversity).
Bob
On 05/08/2013 02:56 PM, Iris Kröger wrote:
Dear list members,
I want to analyse the impact of a competitor community (i.e. community
abundances on the one hand and community species diversity on the other
hand) on mosquito larval populations of species A and B. Each variable on its
own has a negative impact on mosquitoes - but when both variables are
interacting, there is a positive impact... How can I interpret that? For
mosquito A only C.diversity has a significant impact - but the interaction
between C.abundances and C.diversity is significant? What does that mean?
I used the model:
lm (mosquito ~ C.abundances * C.diversity)
output Mosquito A:
Estimate Std. Error t value Pr(|t|)
(Intercept) -0.2120 0.1159 -1.829 0.074855 .
C.abundances -0.1277 0.1616 -0.790 0.434067
C.diversity -0.5787 0.1385 -4.178 0.000155 ***
C.abundances:C.diversity 0.4096 0.1712 2.393 0.021476 *
Output Mosquito B:
Estimate Std. Error t value Pr(|t|)
(Intercept) -0.2900 0.1220 -2.377 0.02233 *
C.abundances -0.3856 0.1701 -2.266 0.02891 *
C.diversity -0.3470 0.1458 -2.381 0.02213 *
C.abundances:C.diversity 0.5367 0.1801 2.980 0.00489 **
Thanks a lot for your help!
Iris
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Bob O'Hara
Biodiversity and Climate Research Centre
Senckenberganlage 25
D-60325 Frankfurt am Main,
Germany
Tel: +49 69 798 40226
Mobile: +49 1515 888 5440
WWW: http://www.bik-f.de/root/index.php?page_id=219
Blog: http://blogs.nature.com/boboh
Journal of Negative Results - EEB: www.jnr-eeb.org
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