An absence of a scale for measuring exposure to the English language, which has a significant effect on English achievement, was detected in the literature. For this reason, in this study, a six-dimensional scale was developed to detect the level of English language exposure and its construct validity was tested. The factor structure of the scale was determined by exploratory factor analysis with the data collected from 784 university students, 726 of whom are undergraduate and 58 of whom are Master's and Ph.D. students. Confirmation of the factor structure of the scale was carried out with a measurement model specified in a structural equation model. A structural equation modeling study was performed along with 233 students from English preparation classes at a university. In the structural model, the effect of exposure to English on the students' scores received from writing in English, speaking in English and the total score (grammar, vocabulary, reading and listening scores) was examined. It was found that exposure to English has a significant effect on all of the three variables. Exposure to English explained the variance of the speaking variable most, while that effect is the least for the writing variable.

In this study, the effects of students’ attitude towards English, foreign language anxiety, exposure to English, school type (i.e. state or private), parents’ English proficiency and education level on the students’ English achievement in TEOG exam are investigated. To achieve this goal, a multiple indicators and multiple causes (MIMIC) model has been developed, and tested based on Stephen Krashen’s language acquisition theory and the literature. According to the research results, attitudes towards English, school type, amount of English exposure, parents’ education level and English proficiency have meaningful and positive effects on English proficiency, while language anxiety has a meaningful but negative effect on it. Moreover, it was found that the effects of parents’ education level and English proficiency and the students’ attitudes towards English on English proficiency are mediated by exposure to English. Therefore, Krashen’s second language acquisition theory stating that those who have positive attitudes towards English will seek more comprehensible input and so will be more exposed to English and consequently will acquire English more is validated.

The Vocabulary Size Test (VST) is one of the most commonly used assessment tools for measuring English vocabulary size in the field of language testing. Despite its common usage, only a limited number of validity and reliability studies were carried out for the VST. Besides, they were mostly predicated on the Rasch model. This validation study attempted to reveal evidence for construct validity for the VST, and for this purpose, item response theory (IRT) analyses were performed based on the three-parameter logistic model (3PLM). The assumptions of IRT were investigated via factor analysis (unidimensionality) and Yen’s Q3 statistic (local independence). To add evidence based on internal structure and to check fairness as lack of measurement bias, detailed differential item functioning (DIF) analyses were conducted with Mantel-Haenszel, Lord's chi-square test, and Logistic regression methods. The validation results with IRT showed that the 3PLM, which includes the guessing parameter, fitted the data better than the one and the two-parameter logistic models. DIF results indicated that 10 items exhibited large DIF (five favoring males and five favoring females). The results further showed that the guessing effect was not negligible for the VST.