A significant number of complex repair jobs carried out by Ace Repairs have to be reworked under the company' s warranty. The reworked jobs are invariably satisfactory. When initial repairs are inadequate, ttherefore, it is not because the mechanics lack competence; rather, there is clearly a level of focused concentration that complex repairs require that is elicited more reliably by rework jobs than by first-time jobs.
The argument above assumes that there is no systematic difference in membership between the group of mechanics who do first-time jobs and the group of those who do rework jobs.