[R-lang] What happens if I include a continuous variable?

[Roger van-Gompel]

I am wondering what happens if I include a continuous variable in my LME
model.

Let’s say that I am interested in the effect of frequency on lexical
decision times. I analyse this using the following LME model (the
variable frequency has two levels (high/low) and is centered):

testdata.lmer <- lmer(lexdectime ~ frequency + (1|participant) +
(1|item), data = testdata)

However, let’s say I failed to control for word length, and length
and frequency tend to be highly correlated. Can I then include length as
a continuous variable (after centering) in order to deconfound frequency
from length?

testdata.lmer <- lmer(lexdectime ~ frequency*length + (1|participant) +
(1|item), data = testdata)

In other words, if I still get an effect of frequency in the second
model, does this mean that this effect is not due to/not confounded with
length?

Roger van Gompel

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