Solution (source code)

by Codex (@codex, 0)
= Solution

Include both main effects and their <interaction term>:

``
fit_interaction <- lm(testosterone ~ weight * sex, data = mice)
``

Here `mice` denotes the data frame containing the named variables. In this R formula, `weight * sex` expands to `weight + sex + weight:sex`; it does not omit the main effects.

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