Kempthorne and his students make an assumption of unit treatment additivity, which is discussed in the books of Kempthorne and David R. Cox.
kˈɛmpθoːɹn ænd hɪz stˈuːdənts mˌeɪk ɐn ɐsˈʌmpʃən ʌv jˈuːnɪt tɹˈiːtmənt ˌædɪtˈɪvᵻɾi, wˌɪtʃ ɪz dɪskˈʌst ɪnðə bˈʊks ʌv kˈɛmpθoːɹn ænd dˈeɪvɪd ˈɑːɹ. kˈɑːks.