The theory suggests that when a procedure cannot be performed, an impasse occurs and the individual applies various strategies to overcome the impasse. Repair theory has been implemented in the form of a computer model called Sierra. Even with simple subtraction problems, many types of bugs were found, often occurring in combinations.
Students shift back and forth among bugs, a phenomenon called bug migration.
The theory's explanation for bug migration is that the student has a stable underlying procedure but that the pro cedure is incomplete in such a way that the student reaches impasses on some problems. Students can apply any repair they can think of. Sometimes they choose one repair and sometimes another. The different repairs manifest themselves as different bugs. So bug migration comes from varying the choice of repairs to a stable, underlying impasse.