The importance of English vocabulary for success in English-speaking schools cannot be overestimated. The authors of the National Reading Panel (2000) wrote: “Benefits in understanding text by applying letter-sound correspondences to printed material come about only if the target word is in the learner’s oral vocabulary, (ch. 4, p. 3)” Chall, a well-known reading scholar, argued that written vocabulary test was effectively equivalent to reading comprehension testing because the correlation between the two was so high (at r=.95 in my own studies), that it is not necessary to test comprehension. Hazenberg and Hulstijn (1996) reported that children with vocabularies of less than 11,000 root words were unable to succeed in college programs. (They noted that this study was done in the Netherlands, and that somewhat higher vocabularies would probably be needed in English.)
A simple example from my own research illustrates the relationship between basic reading skills and vocabulary. When we (Biemiller & Slonim, 2001) conducted our second normative study, we included a simple test of oral reading of 60 words after orally testing vocabulary meanings of the same words. We found that from Grade 3 on, 95% of children could read more words than they could define. Figure 11.1 illustrates this relationship. One regression line in the figure simply shows the level of vocabulary known. The other regression line shows accuracy in reading the words tested for vocabulary. Although the number of words read correctly was correlated with the number of words defined (r=.45, N=92), after Grade 2, most children could read more words than they could explain. The average difference was 25% to 30% more words read correctly than understood (see Table 11.1).