Therefore, by contraposition, a necessary condition for unit-treatment additivity is that the variance is constant.
ðˈɛɹfoːɹ, baɪ kˌɑːntɹəpəzˈɪʃən, ɐ nˈɛsᵻsɚɹi kəndˈɪʃən fɔːɹ jˈuːnɪttɹˈiːtmənt ˌædɪtˈɪvᵻɾi ɪz ðætðə vˈɛɹiəns ɪz kˈɑːnstənt.