The assumption of unit-treatment additivity implies that, for every treatment , the th treatment has exactly the same effect  on every experiment unit.
ðɪ ɐsˈʌmpʃən ʌv jˈuːnɪttɹˈiːtmənt ˌædɪtˈɪvᵻɾi ɪmplˈaɪz ðˈæt, fɔːɹ ˈɛvɹi tɹˈiːtmənt , ðə tˌiːˈeɪtʃ tɹˈiːtmənt hɐz ɛɡzˈæktli ðə sˈeɪm ɪfˈɛkt ˌɔn ˈɛvɹi ɛkspˈɛɹɪmənt jˈuːnɪt.