A word family is the base form of a word plus its inflected forms and derived forms made from affixes (Hirsh & Nation 1992, p. 692). In English language, inflections include third person -s, -ed, -ing, plural -s, possessive -s, comparative -er and superlative -est. Affixes includes -able, -er, -ish, -less, -ly, -ness, -th, -y, non-, un-, -al, -ation, -ess, -ful, -ism, -ist, -ity, -ize, -ment, in- (Hirsh & Nation 1992, p. 692). The idea is that a base word and its inflected forms support the same core meaning, and can be considered learned words if a learner knows both the base word and the affix. Bauer and Nation (Bauer & Nation 1993) proposed seven levels of affixes.
- Hirsh, D.; Nation, P. (1992), "What Vocabulary Size Is Needed to Read Unsimplified Texts for Pleasure?.", Reading in a Foreign Language 8 (2), pp. 689–96
- Bauer, L; Nation, P (1993), "Word Families", (abstract)