An ELT Glossary : Coverage
Coverage is a quality of a test, and refers to the number and appropriacy of the items included. This should be balanced and reflect what the learner has been taught and/or is expected to know. A test might have inadequate coverage if :
a) it was too short overall, so that only a small portion of what needed to be tested could be included. Determining test length therefore has to be a balance between ensuring that the test is long enough to provide adequate coverage, but not so long as to tire / bore /demotivate learners. This is an issue eg with general purpose placement tests, which must contain enough items from each level to discriminate between learners, and ideally would test all four skills as well as systems items such as grammar or lexis. However, as placement tests are frequently used in commercial language schools before enrolment (and are therefore part of the sales stage), they must not be so long as to make the learner decide not to stay for the test.
b) it over-emphasised some items at the expense of others - eg if in an end of course achievement test 70% of the items related to lexis focused on lexis connected with the environment, even though this had only been covered in one out of the ten units of the course, and the test included only 30% of items testing the lexis from other units and the topics covered there.
c) it overemphasised some systems/skills at the expense of others, for example if the test contained only items testing grammar, lexis and reading, while the course had given equal emphasis to the development of listening.
d) it overemphasised some sections of a course at the expense of others - eg a progress test for Units 5-8 of a course, which was made up 80% of items from the final two units and only 20% of items from units 5 and 6.
e) It was too easy or difficult. For example a proficiency test which aimed to demonstrate that learners were at a strong CEFR B2 level, but only included items that would be expected to have been mastered at A2 and B1 with no coverage of items that a B2 level learner would be expected to know.