Thresholds, Text Coverage, Vocabulary Size, and Reading Comprehension in Applied Linguistics

The inextricable link between vocabulary knowledge and reading comprehension is incontrovertible. However, questions remain regarding the nature of the interaction. One question which remains unresolved is whether there is an optimum text coverage, or ratio of known to unknown words in a text, such that any deleterious effects of the unknown words on reading comprehension are minimised. A related question is what vocabulary size would a reader need to have in order to achieve the optimum text coverage for a given text or class of texts.  This thesis addresses these questions in three ways. First, a replication and expansion of a key study (Hu & Nation, 2000)1 was performed. In that study, 98% text coverage was found to be optimal for adequate reading comprehension of short fiction texts when reading for pleasure. To replicate that study, equivalent measures of reading comprehension were collected from a more homogeneous group of participants at a university in northern Thailand (n = 138), under stricter conditions and random assignment to one of three text coverage conditions, to verify the generalisability of the results. The original study was also expanded by measuring reader characteristics thought to contribute to reading comprehension, such as vocabulary size, l1 and l2 literacy, and reading attitudes, in an effort to improve the explainable reading comprehension variance.  In order to more accurately calculate the text coverage a reader experiences for a particular text, both the vocabulary profile of the text and the vocabulary size of the reader must be known as precisely as possible. Therefore, to contribute to the question of vocabulary size, changes such as measuring item completion time and varying the order of item presentation were made to the VST (P. Nation & Beglar, 2007) to improve its sensitivity and accuracy. This may ultimately lead to increased precision when using text coverage to predict reading comprehension.  Finally, l2 English vocabulary size norms were established to supplement the diagnostic usefulness of the VST. Data were collected through an online version of the VST created for this thesis from primarily self-selected participants (n 1:31 105) located in countries (n 100) around the world representing several l1 and age groups.  Analysis of the data collected for this thesis suggest that text coverage explains much less reading comprehension variance than previously reported while vocabulary size may be a more powerful predictor. An internal replication of Hu and Nation (2000) found errors in the calculation of optimum text coverage and in the reported size of the effect on reading comprehension. A critical review of the theoretical foundations of the text coverage model of reading comprehension found serious flaws in construct operationalisation and research design. Due to these flaws, most research which has purported to measure the effect of text coverage on reading comprehension actually measured the effect of an intervening variable: readers’ vocabulary size.  Vocabulary size norms derived from data collected through an online version of the VST appear to be reliable and representative. Varying item presentation order appears to increase test sensitivity. Despite a moderate effect for l1 English users, item completion time does not seem to account for any variance in vocabulary size scores for l2 English learners.  Based on the finding that vocabulary size may explain both reading comprehension and text coverage, the putative power of text coverage to predict reading comprehension is challenged. However, an alternative measure which may offer greater power to predict reading comprehension, the VST, has been modified and made available online. This version of the VST may provide greater sensitivity and ease of use than the offline, paper-based version.