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
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