The assumption of unit treatment additivity usually cannot be directly falsified, according to Cox and Kempthorne.
ðɪ ɐsˈʌmpʃən ʌv jˈuːnɪt tɹˈiːtmənt ˌædɪtˈɪvᵻɾi jˈuːʒuːəli kænˈɑːt biː dᵻɹˈɛktli fˈɔlsᵻfˌaɪd, ɐkˈoːɹdɪŋ tə kˈɑːks ænd kˈɛmpθoːɹn.