Online tests can tell you, 'You know about 6,500 English words!' How do they work? The Vocabulary Size Test asks 10 questions for each group of 1,000 word families. Each correct answer counts as 100 word families. This is a quick way to estimate your vocabulary size.

But with multiple-choice questions, you can sometimes guess the right answer. So your score may be higher than your real knowledge.

The Vocabulary Size Test, created by Paul Nation and his colleagues, divides English words into groups of 1,000 word families according to how often they are used. It asks 10 multiple-choice questions from each group, making 140 questions in total, and each correct answer represents 100 word families. In 2010, Beglar tested 19 native speakers and 178 Japanese learners and found that the test was highly reliable.

However, a 2015 study found that multiple-choice answers may overestimate knowledge because learners can guess or use elimination. In 2021, a paper written with Nation himself concluded that existing tests are not accurate enough for many purposes.

A vocabulary size test turns everything a learner knows about words into one number. How far can that number be trusted? The best-known instrument, Paul Nation's Vocabulary Size Test, takes ten items from each 1,000-word-family frequency band, up to 14,000, and multiplies each correct answer by 100.

In 2010, Beglar checked it with Rasch analysis, testing 19 native speakers and 178 Japanese learners. Most items behaved as predicted, and reliability was very high. Later work found weak spots. A 2015 study compared multiple-choice answers with other evidence of word knowledge and found mismatches.

Guessing and elimination, the authors noted, can push scores up, and precise measurement would take more items. In 2021, Stoeckel, McLean and Nation himself listed three problems. The multiple-choice format leads to overestimation. The test assumes that knowing one word means knowing its whole family. And each band has only a few items.

Existing tests, they concluded, are not precise enough for many purposes. These tests also check only whether a learner recognizes a meaning. Whether the learner can use the word is a separate question. For comparison, Brysbaert and colleagues estimated that a 20-year-old American native speaker knows about 42,000 lemmas, roughly 11,000 word families.