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The idea of a word family is important for a systematic approach to vocabulary teaching and for deciding the vocabulary load of texts. Inclusion of a related form of a word within a word family depends on criteria involving frequency, regularity, productivity and predictability. These criteria are applied to English affixes so that the inflectional affixes and the most useful derivational affixes are arranged into a graded set of seven levels. This set of levels and others like it have value in guiding teaching and learning, in standardising vocabulary load and vocabulary size research, in investigating lexical development and lexical storage, and in guiding dictionary making.
Bauer et al. (Fri,) studied this question.