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The relationship between vocabulary knowledge and text readability is a robust one. Vocabulary knowledge has consistently been found to be the foremost predictor of a text's difficulty. However, the relationship is a complex one. Procedures used by readability formulae to assess the vocabulary factor can over- or underestimate text difficulty. Counting syllables ignores difficult monosyllable words or misses difficult idiomatic expressions. Counts of word frequency can ignore the effects of affixes. In general, it is not the mechanical counts of “easy” or “difficult” words in a text that make a text easy or difficult, but what the reader knows about the words in a text.