School systems and social services agencies continue to grapple with personnel evaluation problems. One way to make personnel evaluation more useful is to improve the validity of evaluation judgments in appraising work performance. This paper proposes to: (1) review literature providing insight into evaluators' reasons for making inaccurate judgments concerning others' work performance; (2) describe common evaluation errors; and (3) provide guidelines for making evaluators' assessments more valid. A literature review identifies four major impediments to obtaining valid performance evaluations: cognitive processes (categorization and memory); innate forces (desire to be accepted, concern with self-protection and success, and affiliation needs); evaluators' personal characteristics; and limited observational opportunities. Errors commonly made by evaluators include the halo effect, evaluation bias and inconsistency, first impression or primacy error, performance dimension order, spillover effect, reliance on similarities, central tendency, and negativity effect. One approach to improving the personnel evaluation process is through training sessions. Better assessments can be obtained by coupling peer evaluations with evaluators' assessments or by using evaluation teams. Evaluator behavior affects the way the persons evaluated perceive assessment outcomes. The more frequently evaluators use observations and conferences in their appraisals, the harder it is to deny variability in personal performance or evaluator ability to judge competence. Developing research-based criteria can help reduce district biases. Evaluation systems that involve evaluators and evaluatees in the design process enhance the credibility and perceived validity of the evaluation process. (54 references) (MLH)