At what rate do learners learn and retain new vocabulary from reading a graded reader?

Abstract This study examined the rate at which vocabulary was learned from reading the 400 headword graded reader A Little Princess. To ascertain whether words of different frequency of occurrence rates were more likely to be learned and retained or forgotten, 25 words within five bands of differing frequency of occurrence (15 to 18 times to those appearing only once) were selected. The spelling of each word was changed to ensure that each test item was unknown to the 15 intermediate level (or above) female Japanese subjects. Three tests (word-form recognition, prompted meaning recognition and unprompted meaning recognition) were administered immediately after reading, after one week and after a three month delay. The results show that words can be learned incidentally but that most of the words were not learned. More frequent words were more likely to be learned and were more resistant to decay. The data suggest that, on average, the meaning of only one of the 25 items will be remembered after three months, and the meaning of none of the items that were met fewer than eight times will be remembered three months later. The data thus suggest that very little new vocabulary is retained from reading one graded reader, and that a massive amount of graded reading is needed to build new vocabulary. It is suggested that the benefits of reading a graded reader should not only be assessed by researching vocabulary gains and retention, but by looking at how graded readers help develop and enrich already known vocabulary.
Keywords: guessing vocabulary from context, vocabulary acquisition, graded readers, occurrence rate, vocabulary decay, vocabulary attrition, extensive reading
Introduction
Second language and reading development It is received wisdom that people learn most of their vocabulary from reading (e.g., Sternberg, 1987). Others take this a little further. Krashen, for example, states that "reading is good for  you. The research supports a stronger conclusion, however. Reading is the only way, the only way we become good readers, develop a good writing style, an adequate vocabulary, advanced grammar, and the only way we become good spellers" (1993:23). In either case, reading is seen to be beneficial for foreign language learning and especially for vocabulary building. There are now quite a number of studies which have looked at how much vocabulary is learned from reading in a foreign language. Examples include, Day, Omura and Hiramatsu (1991); Dupuy and Krashen (1993); Grabe and Stoller (1997); Hayashi (1999); Horst, Cobb and Meara (1998); Mason and Krashen (1997) and Pitts, White and Krashen, (1989) among others. The general picture that emerges from these studies is that learners do learn vocabulary from their reading but not very much. In many of the studies, typically the gains in scores after reading are only just significant and not much better than random guessing on the tests. Table 1 provides a representative sample of some of the more commonly cited research that has looked at the amount of vocabulary learned from reading in a foreign language. These studies seem to show modest but positive gains in vocabulary acquisition from reading in a foreign language. However, most studies are rather short and the text was often quite difficult.