How Reliable are TOEFL Scores?

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Abstract
The reliability of scores on four forms of the Test of English as a Foreign Language (TOEFL) was estimated using a hybrid IRT model. It was found that there was very little difference between their overall reliability when the testlet items were assumed to be independent and when their dependence was modeled. A larger difference in reliability was found when test sections were analyzed individually. Then we found as much as a 40% overestimate in reading comprehension testlets, with the longer testlets of the newest form of TOEFL showing the most local dependence. The listening comprehension testlets exhibited much less local dependence. We also found that the test was unidimensional enough for the use of univariate item response theory (IRT) to be efficacious, and that the reading comprehension testlets showed essentially no differential functioning by sex.