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

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.