Advantage:
It permits a description of some systematic aspects of learner language.
Constraints:
It does not always give us clear insights into what causes learners to do what they do, because
It is very often difficult to determine the source of errors.
Learners sometimes avoid using certain features of language which they perceive difficult. The avoidance of particular features will be difficult to observe, but it may also be a part of the learner’s systematic L2 performance