Interpreting interaction coefficient in R (Part1 lm) UPDATED
(going through this post again three years after I posted it. Made some, hopefully useful, changes)
Interaction are the funny interesting part of ecology, the most fun during data analysis is when you try to understand and to derive explanations from the estimated coefficients of your model. However you do need to know what is behind these estimate, there is a mathematical foundation between them that you need to be aware of before being able to derive explanations.
I plan to make two post on this issue, this first one will deal with interpreting interactions coefficients from classical linear models, a second one (which never came out) will look at the Fratios of these coefficients and what they mean. I will only look at twoway interaction because above this my brain start to collapse. Some later post might be taking into account the extensive literature on these issues that I only started to scratch.
If you want to have a look at a clean page with code/figures go there: http://rpubs.com/hughes/15353
This post is divided in three parts: i) interaction between two categorical variables, ii) interaction between one continuous and one categorical variables and finally iii) interaction between two continuous variables.
Interaction between two categorical variables:
Let’s make an hypothetical examples: say we measured the shoot length (in cm) of some plant species under two different treatments: a temperature treatment with 2 levels (Control and Elevated temperature) and a nutrient treatment with three levels of nitrogen (Control, Medium, High nitrogen concentration). Following the recommendation of our stats professor we made a fullyfactorial design and aim at look at the effect of these two treatments and their interactions on the shoot length.
#interpreting interaction coefficients from lm
#first case two categorical variables
set.seed(12)
f1<gl(n=2,k=30,labels=c("Low","High"))
f2<as.factor(rep(c("A","B","C"),times=20))
modmat<model.matrix(~f1*f2,data.frame(f1=f1,f2=f2))
coeff<c(1,3,2,4,1,1.2)
y<rnorm(n=60,mean=modmat%*%coeff,sd=0.1)
dat<data.frame(y=y,f1=f1,f2=f2)
summary(lm(y~f1*f2))
#output
#Call:
#lm(formula = y ~ f1 * f2)
#Residuals:
# Min 1Q Median 3Q Max
#0.199479 0.063752 0.001089 0.058162 0.222229
#Coefficients:
# Estimate Std. Error t value Pr(>t)
#(Intercept) 0.97849 0.02865 34.15 <2e16 ***
#f1High 3.00306 0.04052 74.11 <2e16 ***
#f2B 1.97878 0.04052 48.83 <2e16 ***
#f2C 4.00206 0.04052 98.77 <2e16 ***
#f1High:f2B 0.98924 0.05731 17.26 <2e16 ***
#f1High:f2C 1.16620 0.05731 20.35 <2e16 ***
#
#Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
#
#Residual standard error: 0.09061 on 54 degrees of freedom
#Multiple Rsquared: 0.9988, Adjusted Rsquared: 0.9987
#Fstatistic: 8785 on 5 and 54 DF, pvalue: < 2.2e16
Let’s go through each coefficient:

(Intercept): plant shoots are predicted to have a length of 0.98cm under control temperature and nitrogen conditions, this is our baseline.

f1High: plant shoots grow 3cm bigger under elevated temperature and control nitrogen conditions compared to the baseline

f2B: plant shoots grow ~2cm smaller under medium nitrogen and control temperature condition compared to the baseline

f2C: plant shoots grow 4cm smaller under high nitrogen and control temperature condition compared to the baseline

f1High:f2B : effect of medium nitrogen condition in plant shoots under elevated temperature increase by approx. 1 compared to the effect of medium nitrogen condition under control temperature conditions. In other words, when the effect of adding medium nitrogen was to decrease plant shoots length by ~2cm in control temperature condition, in elevated temperature conditions adding medium level of nitrogen lead to shoot length of 0.97 1.97 + 3 + 0.98 = 3 on shoot length.

f1High: f2C : same as above for high nitrogen condition.
The best way to grasp what these means is still to make a quick graph:
Not as easy to understand as one would thought …
Interaction between one continuous and one categorical variables
Say that we are weighting soil organisms in a field experiment where we added a temperature treatment with two levels (Control, Elevated) and we measured the soil nitrogen concentration (continuous variable measured in say mg per gram of soil), we would like to see the effects of the nitrogen concentration and its interaction with temperature on the biomass of soil organisms.
#second case one categorical and one continuous variable
x<runif(50,0,10)
f1<gl(n=2,k=25,labels=c("Low","High"))
modmat<model.matrix(~x*f1,data.frame(f1=f1,x=x))
coeff<c(1,3,2,1.5)
y<rnorm(n=50,mean=modmat%*%coeff,sd=0.5)
dat<data.frame(y=y,f1=f1,x=x)
summary(lm(y~x*f1))
#ouput
#Call:
#lm(formula = y ~ x * f1)
#
#Residuals:
# Min 1Q Median 3Q Max
#0.9627 0.2945 0.1238 0.3386 0.9835
#Coefficients:
# Estimate Std. Error t value Pr(>t)
#(Intercept) 1.35817 0.17959 7.563 1.31e09 ***
#x 2.95059 0.03187 92.577 < 2e16 ***
#f1High 2.63301 0.25544 10.308 1.54e13 ***
#x:f1High 1.59598 0.04713 33.867 < 2e16 ***
#
#Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
#Residual standard error: 0.4842 on 46 degrees of freedom
#Multiple Rsquared: 0.9983, Adjusted Rsquared: 0.9981
#Fstatistic: 8792 on 3 and 46 DF, pvalue: < 2.2e16
Again let’s go through the coefficients:

(Intercept): Under control temperature condition and with a nitrogen concentration of 0 mg/g, soil organism biomass is 1.36 mg/g

x: Adding 1mg/g of nitrogen increase soil organism biomass by 2.95 mg/g in control temperature conditions

f1High: Under elevated temperature condition and for a nitrogen concentration of 0 mg/g soil organism biomass is 2.63 mg/g lower compared to control temperature condition

x:f1High : the effect of soil nitrogen on soil organism biomass is higher by 1.59 under elevated temperature condition compared to control condition. In other words the slope between soil nitrogen and soil organism biomass is 2.95 + 1.59 = 4.54 in elevated temperature condition.
And a graph to grasp:
Interaction between two continuous variables
Now for the last possible case imagine that we wanted to compare our experimental results to what is happening in the “real” world. We went happily outside measured air temperature at the soil surface and collected soil samples from which we got soil nitrogen concentration (in mg per g of soil) and soil organism biomass (in mg per g of soil). Again we build a linear model to test the effect of temperature and nitrogen plus their interaction.
<code class="r"><span class="comment"># third case interaction between two continuous variables</span>
<span class="identifier">x1</span> <span class="operator"><</span> <span class="identifier">runif</span><span class="paren">(</span><span class="number">50</span>, <span class="number">0</span>, <span class="number">10</span><span class="paren">)</span>
<span class="identifier">x2</span> <span class="operator"><</span> <span class="identifier">rnorm</span><span class="paren">(</span><span class="number">50</span>, <span class="number">10</span>, <span class="number">3</span><span class="paren">)</span>
<span class="identifier">modmat</span> <span class="operator"><</span> <span class="identifier">model.matrix</span><span class="paren">(</span><span class="operator">~</span><span class="identifier">x1</span> <span class="operator">*</span> <span class="identifier">x2</span>, <span class="identifier">data.frame</span><span class="paren">(</span><span class="identifier">x1</span> <span class="operator">=</span> <span class="identifier">x1</span>, <span class="identifier">x2</span> <span class="operator">=</span> <span class="identifier">x2</span><span class="paren">)</span><span class="paren">)</span>
<span class="identifier">coeff</span> <span class="operator"><</span> <span class="identifier">c</span><span class="paren">(</span><span class="number">1</span>, <span class="number">2</span>, <span class="operator"></span><span class="number">1</span>, <span class="number">1.5</span><span class="paren">)</span>
<span class="identifier">y</span> <span class="operator"><</span> <span class="identifier">rnorm</span><span class="paren">(</span><span class="number">50</span>, <span class="identifier">mean</span> <span class="operator">=</span> <span class="identifier">modmat</span> <span class="operator">%*%</span> <span class="identifier">coeff</span>, <span class="identifier">sd</span> <span class="operator">=</span> <span class="number">0.5</span><span class="paren">)</span>
<span class="identifier">dat</span> <span class="operator"><</span> <span class="identifier">data.frame</span><span class="paren">(</span><span class="identifier">y</span> <span class="operator">=</span> <span class="identifier">y</span>, <span class="identifier">x1</span> <span class="operator">=</span> <span class="identifier">x1</span>, <span class="identifier">x2</span> <span class="operator">=</span> <span class="identifier">x2</span><span class="paren">)</span>
<span class="identifier">summary</span><span class="paren">(</span><span class="identifier">lm</span><span class="paren">(</span><span class="identifier">y</span> <span class="operator">~</span> <span class="identifier">x1</span> <span class="operator">*</span> <span class="identifier">x2</span><span class="paren">)</span><span class="paren">)
</span></code>
<code>##Call:
##lm(formula = y ~ x1 * x2)
##
##Residuals:
## Min 1Q Median 3Q Max
##0.68519 0.19426 0.03194 0.18262 0.71513
##
##Coefficients:
## Estimate Std. Error t value Pr(>t)
##(Intercept) 0.965921 0.284706 3.393 0.00143 **
##x1 1.995115 0.049260 40.502 < 2e16 ***
##x2 0.993288 0.027835 35.685 < 2e16 ***
##x1:x2 1.499595 0.004651 322.443 < 2e16 ***
##
##Signif. codes:
##0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
##
##Residual standard error: 0.3264 on 46 degrees of freedom
##Multiple Rsquared: 1, Adjusted Rsquared: 1
##Fstatistic: 4.661e+05 on 3 and 46 DF, pvalue: < 2.2e16</code>
The coefficient means:

(Intercept): at 0°C and with nitrogen concentration of 0 mg/g, soil organism biomass is 0.96 mg/g

x1: increasing temperature by 1°C at a nitrogen concentration of 0mg/g increase soil organism biomass by ~2 mg/g

x2: increasing nitrogen concentration by 1 mg/g at a temperature of 0°C reduce the soil organism biomass by ~1 mg/g

x1:x2 : (sit down and breath deeply) the effect of temperature on soil organism biomass increase by 1.49 for every unit increase in soil nitrogen. For example at soil nitrogen concentration of 0 mg/g the slope between soil organism biomass and temperature is ~2, at soil nitrogen concentration of 1mg/g this slopes is: 2 + 1.49 = 4.49. This also works the other way round (ie effect of nitrogen on soil organism biomass become more positive as temperature increase).
And a final graph:
In this graph we drew the regression line of the relation between soil organism and soil nitrogen at three different temperature: at 0, 5 and 10°C.
So next time you want to fit 5way interactions try to remember that already understanding twoway interaction need some thinking …
Happy coding.
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