Employees should be appraised periodically. Most organization have annual appraisal systems. The formal annual evaluation should not be a substitute for periodic informal employee appraisal. The periodic appraisal gives the employee the opportunity to correct deficiencies before the formal annual evaluation. The main purpose of an evaluation is not punitive but corrective.
The "periodic" appraisal can assist the supervisor in judging the degrees of trust he can place in his subordinate and trust in his punctual and effective performance.