Dear Iris,
The expected value of any treatment combination can be calculated as sum
of the coefficients. For example:
E(no comptetitors, no insecticide) = Intercept
E(many competitors, high concentration) = Intercept + conditionold +
contaminationhigh + conditionold:contaminationhigh
The non-significant interactions would mean that effects of competition
and insecticide are simple summed.
You have two interaction parameters that significantly differ from zero.
For example, conditionold:contaminationhigh=1.2005231. This value can be
interpreted by two ways. High insecticede concentration has a strong
negative effect if no competion (contaminationhigh=-1.2825250), but it
has negligible effect, if competition is stong:
-1.2825250+1.2005231=-0.0820019. The interaction parameter measures the
difference between two levels of competition in the effect of insecticide.
Another interpretation is that interaction coefficient measures the
difference between two levels of insecticide treatment in the effect of
strong competition (ie. in the decrease of abundance caused by strong
competition).
From mathematical point of view these two interpretations are
equivalent. You can choose either.
Best wishes
Zoltan
2013.01.29. 14:54 keltezéssel, "Iris Kröger" írta:
Dear R-Sig-Ecology,
I'm analysing an experiment about mosquito larvae beeing treated with 3 conditions (no competitors added = "algae", few competitors added
="new", many competitors added ="old"). After 2 weeks I contaminated all conditions with 3 insecticide concentrations (no
treatment = "control", low concentration ="low", medium concentration = "medium", high concentration
="high"). The effect of treatments was observed over a time period of 35 days.
Now I would like to analyse, if there is a synergistic effect of condition and
insecticide treatment. I used a gls model (see below).
(Important: the mosquito larval abundances differed significantly between
conditions when insecticide contamination was applied. I'm not sure if this is
a problem for the further analyses....)
Obviously there is a synergistic effect (condition * contamination, p<0.001), but I
don't know how to interpret the output. Which treatments did the model compare?Does it
mean, that the effect of medium and high contamination increased significantly at
old-condition compared with the effect at algae-condition? (This I can hardly believe.
Mosquito larval abundances were close to zero even without contamination at
condition-old&contamination-control , hence an additional effect of insecticide
treatment should be hardly detectable. I would rather expect an effect at
condition-new in combination with contamination...)
I used:
mod1<-gls(log10(mosquito+1) ~condition * contamination , data=MK ,
correlation = corAR1(form=~day |bucket),
weights=varIdent(form= ~ 1|condition),
method="REML")
summary(mod1)
anova(mod1)
The output was:
anova(mod5)
Denom. DF: 258
numDF F-value p-value
(Intercept) 1 221.04772 <.0001
condition 2 243.79461 <.0001
contamination 3 24.71058 <.0001
condition:contamination 6 23.92481 <.0001
Coefficients:
Value Std.Error t-value p-value
(Intercept) 1.9778939 0.1391071 14.218493 0.0000
conditionnew -0.7766661 0.1610017 -4.823963 0.0000
conditionold -1.8958920 0.1461519 -12.972068 0.0000
contaminationhigh -1.2825250 0.1840214 -6.969433 0.0000
contaminationlow -0.2044417 0.1840214 -1.110967 0.2676
contaminationmedium -0.7833268 0.1840214 -4.256715 0.0000
conditionnew:contaminationhigh 0.1011773 0.2129852 0.475044 0.6352
conditionold:contaminationhigh 1.2005231 0.1933408 6.209364 0.0000
conditionnew:contaminationlow -0.1917487 0.2129852 -0.900291 0.3688
conditionold:contaminationlow 0.1496570 0.1933408 0.774058 0.4396
conditionnew:contaminationmedium -0.1454105 0.2129852 -0.682726 0.4954
conditionold:contaminationmedium 0.7940461 0.1933408 4.106978 0.0001
Can anyone help me?
Many thanks,
Iris
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--
Botta-Dukát Zoltán
--------------------------------
Ökológiai és Botanikai Intézet
Magyar Tudományos Akadémia
Ökológiai Kutatóközpont
--------------------------------
2163. Vácrátót, Alkotmány u. 2-4.
tel: +36 28 360122/157
fax: +36 28 360110
botta-dukat.zol...@okologia.mta.hu
www.okologia.mta.hu
Zoltán BOTTA-Dukát
--------------------------------
Institute of Ecology and Botany
Hungarian Academy of Sciences
Centre for Ecological Research
--------------------------------
H-2163 Vácrátót, Alkomány u. 2-4.
HUNGARY
Phone: +36 28 360122/157
Fax..: +36 28 360110
botta-dukat.zol...@okologia.mta.hu
www.okologia.mta.hu
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