How Many Words Do We Know? Practical Estimates of Vocabulary Size Dependent on Word Definition, the Degree of Language Input and the Participant’s Age

When one looks at lists of lemmas, it rapidly becomes clear that they still contain a lot of redundancy, as shown in Table 2 Words form families based on derivation and compounding. Knowledge of one word from a family helps to understand the meaning of the other members (although it may not be enough to produce the word) and to learn these words.

The power of morphological families has been investigated most extensively in second language education (Goulden et al., 1990), where it was observed that participants were often able to understand the meaning of non-taught members of the family on the basis of those taught. If you know what ‘diazotize’ means, you also know what ‘diazotization, diazotizable, and diazotizability’ stand for, and you have a pretty good idea of what is referred to with the words ‘misdiazotize, undiazotizable, or rediazotization.’ Similarly, if you know the adjective ‘effortless,’ you will understand the adverb ‘effortlessly’ (you can even produce it). You will also quickly discover that the adjective ‘effortless’ consists of the noun ‘effort’ and the suffix ‘-less.’ And if you know the words ‘flower’ and ‘pot,’ you understand the meaning of ‘flowerpot.’

Psycholinguistic research (Schreuder and Baayen, 1997; Bertram et al., 2000) has also pointed to the importance of word family size in word processing. Words that are part of a large family (‘graph, man, work, and fish’) are recognized faster than words from small families (‘wren, vase, tumor, and squid’).

Goulden et al. (1990) analyzed the 208,000 distinct lemma entries of Webster’s Third New International Dictionary (1961) described above. Of these, they defined 54,000 as base words (referring to word families), 64,000 as derived words, and 67,000 as compound words (there were also 22,000 lemmas that could not be classified). The person, who has spent most energy on defining word families, is Nation (e.g., Nation, 2006). His current list has pruned (and augmented) Goulden et al.’s (1990) list of 54,000 base words to some 28,000 word families, of which 10,000 suffice for most language understanding.

As is the case with all natural categories, the boundary between base words and derived words is not clear (see Bauer and Nation, 1993, for guidelines). Although most of the time the categorization is straightforward, there are a few thousand borderline instances. For instance, what to do with the trio ‘abbey, abbot, and abbess’? They are not morphologically related according to present-day morphological rules, but they are historically related and the similarity in meaning and orthography helps to understand and learn the three of them. A similar question can be asked about ‘move and motion’: Are they part of the same family or not, given their small orthographic and phonological similarity? Authors disagree which of these are base words and which not, leading some scholars to conclude that the transition from lemmas to word families creates more confusion than clarity (Schmitt, 2010). Therefore, it is interesting to do the counts for both definitions.