Why test vocabulary?
Similar reasons may be advanced for testing vocabulary in proficiency tests to those used to support the inclusion of a grammar section (though vocabulary has its special sampling problems). However, the arguments for a separate component in other kinds of test may not have the same strength. One suspects that much less time is devoted to the regular, conscious teaching of vocabulary than to the similar teaching of grammar. If there is little teaching of vocabulary, it may be argued that there is little call for achievement tests of vocabulary. At the same time, it is to be hoped that vocabulary learning is taking place. Achievement tests that measure the extent of this learning (and encourage it) perhaps do have a part to play in institutional testing. For those who believe that systematic teaching of vocabulary is desirable, vocabulary achievement tests are appreciated for their backwash effect.
The usefulness (and indeed the feasibility) of a general diagnostic test of vocabulary is not readily apparent. As far as placement tests are concerned, we would not normally require, or expect, a particular set of lexical items to be a prerequisite for a particular language class. All we would be looking for is some general indication of the adequacy of the student’s vocabulary. The learning of specific lexical items in class will rarely depend on previous knowledge of other, specified items. One alternative is to use a published test of vocabulary. The other is to construct one’s own vocabulary proficiency test.