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

To supplement the existing estimates, we ran a new study on a much larger scale, both in terms of words tested and in terms of people tested.

Stimulus List

As the authors before us, we rapidly came to the conclusion that not all words in the Webster dictionary and the Collins scrabble lists are of interest, because they include many names of plants, insects, and chemical substances, which come close to proper nouns for non-specialists. In addition, creating a stimulus list on the basis of copyright protected sources creates a problem for free distribution of the materials, which is needed if one wants science to be cumulative.

To tackle the above issues, we decided to build a new list of lemmas ‘worthwhile to be used in psycholinguistic experiments,’ based on word frequency lists (in particular the SUBTLEX lists from Brysbaert et al., 2012 and Van Heuven et al., 2014) and free lists of words for spell checkers. The main criterion for inclusion was the likely interest of non-specialists in knowing the word. Compared to the Collins scrabble list, the list does not include the lemmas ‘aa, aah, aal, allii, aargh, aarrgh, aarrghh, aarti, aasvogel, ab, aba, abac, abacterial, abactinal, abactor, …, zymolysis, zymome, zymometer, zymosan, zymosimeter, zymosis, zymotechnic, zymotechnical, zymotechnics, zymotic, zymotically, zymotics, zythum, zyzzyva, and zzz.’ It does, however, include the words ‘a, aardvark, aardwolf, abaca, aback, abacus, …, zymogen, zymology, and zymurgy.’ All in all, the list contains 61,800 entries and is attached as Supplementary Materials to the present article. Although the list was compiled for research purposes, we are fairly confident it contains the vast majority of reasonably known English words (in American English spelling), but we agree that it would be possible to add about the same number of more specialist words.

To see where we would get and to go for the maximum difference between the lemma list and the word family list, we decided to interpret the family size of our 61,800 lemma list maximally. That is, all words that could reasonably be derived from a base word were considered to be part of the base word’s family. Compound words were split in their constituting families, unless the meaning of the compound could not be derived from the constituents (as in ‘honeymoon, huggermugger, and jerkwater’). This resulted in 18,269 word families (see the Supplementary Materials). With less strict criteria, the list could easily be enhanced to 20,000 families or even 25,000. On the other hand, while reading the list of families, one is constantly tempted to prune even further (so that a list of 18,000 may be achievable as well). The basic finding, however, is that English words boil down to a list of building blocks not much larger than 20,000 words, when names and acronyms (which are often names as well) are excluded. The rather small number of word families is testimony to the tremendous productivity of language.

Participants and the Vocabulary Test Used

To see how many of our list of 61,800 lemmas are known, we presented them to participants with a test similar to the one used by Goulden et al. (1990). Because of the massive internet availability nowadays, it was possible to present the entire list of words to a much larger and heterogeneous group of people (Keuleers et al., 2015).

The test we ran follows the design described in Keuleers et al. (2015). First, participants were asked a few questions, the most important of which for the present discussion were about their age, whether English was their native language, and whether they took higher education. After answering the questions, the participants were shown a random list of 67 words and 33 non-words and asked to indicate which words they knew. The non-words were generated with the Wuggy algorithm (Keuleers and Brysbaert, 2010). Participants were warned about the non-words and they were told they would be punished for yes-responses to these letter strings. To correct the scores for guessing, the proportion of yes-responses to the non-words was subtracted from the proportion of yes-responses to the words. So, someone who selected 50/67 words and said yes to 1/33 non-words, obtained a score of 0.746 - 0.033 = 0.713, indicating that they knew 71.3% of the words. Across participants, data were collected for the full list of 61,800 lemmas and over 100,000 non-words.

Participants taking part in the vocabulary test consented to their data being used for scientific analyses at the word level. At no point they were asked to identify themselves, so that data gathering and data use were not linked to individuals. Participation was voluntary, did not put strain on the participants, and could be stopped at any time. This procedure is in line with the General Ethical Protocol followed at the Faculty of Psychology and Educational Sciences of Ghent University.

All in all, we tested 221,268 individuals who returned 265,346 sessions. The number of sessions is higher than the number of participants because quite a few participants took the vocabulary test more than once (each session had a different sample of words and non-words). In order not to give undue weight to individuals (some participants took the test more than 100 times), we limited the analyses to the first three sessions if participants completed multiple sessions.