In simultaneous interpretation, the interpreter’s or translator’s version may depart from the original or speaker’s version in three general ways: the translator may omit some material from the original version, add some material, or substitute material which, if it is at considerable variance with the original version, may constitute an ‘error’ of translation. The author presents here data relating to the incidence of these events and their relationship to various characteristics of the input materials, i.e. the texts being translated.