In its simplest form, the assumption of unit-treatment additivity states that the observed response  from experimental unit  when receiving treatment  can be written as the sum of the unit's response  and the treatment-effect , that is
ɪn ɪts sˈɪmpəlɪst fˈɔːɹm, ðɪ ɐsˈʌmpʃən ʌv jˈuːnɪttɹˈiːtmənt ˌædɪtˈɪvᵻɾi stˈeɪts ðætðɪ əbzˈɜːvd ɹᵻspˈɑːns fɹʌm ɛkspˌɛɹɪmˈɛntəl jˈuːnɪt wɛn ɹᵻsˈiːvɪŋ tɹˈiːtmənt kæn biː ɹˈɪʔn̩ æz ðə sˈʌm ʌvðə jˈuːnɪts ɹᵻspˈɑːns ænd ðə tɹˈiːtməntɪfˈɛkt , ðæt ˈɪz