 This idea is part of the Rubin Causal Inference Model, developed by Donald Rubin in collaboration with Paul Rosenbaum in the early 1970s.The exact definition differs between their articles in that period.In one of Rubins articles from 1978 Rubin discuss "ignorable assignment mechanisms", which can be understood as the way individuals are assigned to treatment groups is irrelevant for the data analysis, given everything that is recorded about that individual.