Tim Watkins examined the swimming speed of tadpoles as a function of the water temperature, and the water temperature at which the tadpoles had been raised. Since size is a major determinant of speed, the tadpole's length was measured as well.
data(Tadpoles)
A data frame Trucking_jobs with 129 rows and 11 variables.
group Whether the tadpole was raised in cold water ("c") or warm ("w").
rtemp The temperature (C) of the water in which the swimming speed was measured. (The swimming channel is called a "race".)
length The tadpole's length, in mm.
vmax The maximum swimming speed attained in one trial, in mm/sec.
It was hypothesized that tadpoles would swim faster in the temperature of water close to that in which they had been raised.
mod_1 = lm(vmax ~ poly(rtemp, 2) * group + length, data = Tadpoles)