The question has been raised whether various measures of language proficiency that concentrate on different aspects of language are equivalent. Studies have been conducted by various agencies comparing language proficiency assessment instruments. Of interest here are the BINL, LAS, and BSM tests. Because each test has a different set of criteria and each produces different sets of limited English proficient students, language proficiency is distinguished from language abilities. Language abilities are defined as the abilities a speaker possesses to use a language; language proficiency is defined as a quantitative measure of these abilities. Most language proficiency assessment tests focus on phonology, the linguistic components of morphology, syntax, and lexicon. Of interest here is that each instrument has a different specific focus on the features and on the value it assigns to any one feature. Examination of the tests suggests the question of whether different linguistic components are commensurate, and the implications of a positive or negative correlation among the components. An empirical study is described which showed that the linguistic abilities tapped by various tests are non-comparable because different linguistic abilities have different patterns of development and use. (AMH)