Abstract
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.
Not to perambulate the corridors in the hours of repose in the boots of ascension
The above sign is supposedly a request made by a Swiss hotel for guests not to wear hiking boots while others are sleeping. Its humor comes from the obviously mistranslated words used to represent simple ideas. The words that we know help us gain a deeper understanding of the world around us. Being able to understand the differences between words like "perambulate" and "walk" helps us understand the subtleties of both text and conversation.
Vocabulary knowledge is strongly correlated to reading comprehension, so closely correlated that some researchers use the two almost synonymously (Carver, 1999; Thorndike, 1978). Correlations between measures of vocabulary and reading comprehension routinely are in the 0.90s. The correlations have been found to be robust almost regardless of the measures used or the populations tested. Although these measures are of reading comprehension, there certainly would be similar correlations between language comprehension and production and vocabulary. Knowledge of word meanings affects every aspect of language knowledge.
Readability measurement is a research tradition that goes back to the beginnings of the 20th century. Readability research generally produces formulae that purport to be able to estimate the relative difficulty of a passage by a combination of factors. Readability formulae are fairly rough guides to text difficulty, with most formulae having large standard errors of measurement of a full grade level or more (Chall, 1995; Zakaluk & Samuels, 1988). Usually the two factors measured by the formulae are a word factor and a sentence length factor (Chall, 1958; Harrison, 1980). Word difficulty is assumed to be a cause of comprehension difficulty in these formulae, although what we consider to be "word difficulty" may actually be a reflection of something else.