  In the popular debate over the use of ranked-choice voting, it is often
claimed that the method of single transferable vote (STV) is immune or mostly
immune to the so-called ``spoiler effect,'' where the removal of a losing
candidate changes the set of winners. This claim has previously been studied
only in the single-winner case. We investigate how susceptible STV is to the
spoiler effect in multiwinner elections, where the output of the voting method
is a committee of size at least two. To evaluate STV we compare it to numerous
other voting methods including single non-transferable vote, $k$-Borda, and the
Chamberlin-Courant rule. We provide simulation results under three different
random models and empirical results using a large database of real-world
multiwinner political elections from Scotland. Our results show that STV is not
spoiler-proof in any meaningful sense in the multiwinner context, but it tends
to perform well relative to other methods, especially when using real-world
ballot data.
