Percentage of Unknown Vocabulary Words in Text as a Function of the Relative Difficulty of the text: Implications for Instruction

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Abstract
The relationship between the relative difficulty of passages and the number of unknown words in passages was investigated. In Study I, 219 students in Grades 3, 4, 5, and 6 were given 100-word passages sampled from textbooks and library books (trade books) and then asked to underline each word that they did not know. In Study II, 60 graduate students were given a similar task involving 120 passages at the following levels of difficulty: junior high, senior high, college, and graduate school. Relative difficulty (or relative easiness) was determined from the difference between a measure of the reading ability of the student in grade equivalent (GE) units and a measure of the difficulty level of the material in GE units. The results from both Study I and Study II can be summarized as follows: (a) when the material is relatively easy then close to 0% will be unknown basic words, (b) when the material is relatively hard, then 2% or more will be unknown basic words, and (c) when the difficulty of the material is matched closely to the ability of the reader, then around 1% will be unknown basic words. These findings do not support the theory that free reading results in large vocabulary growth because free reading is likely to involve relatively easy material that contains few, if any, unknown words. Indeed, these findings call into question the practice of devoting large amounts of classroom time to free reading if the purpose is to increase vocabulary or reading level.