J. Charles Alderson, 1995. Language Test Construction and Evaluation (Cambridge Language Teaching Library). Edition. Cambridge University Press. P12
Diagnostic tests seek to identify those areas in which a student needs further help. These tests can be fairly general, and show, for example, whether a student needs help with one of the four main language skills; or they can be more specific, seeking perhaps to identify weaknesses in a students use of grammar. These more specific diagnostic tests are not so easy to design since it is difficult to diagnose precisely strengths and weaknesses in the complexities of language ability. For this reason there are very few purely diagnostic tests. However, achievement and proficiency tests are themselves frequently used, albeit unsystematically, for diagnostic purposes.