@SarahGen-- Essay evaluation is a type of individual evaluation but the evaluator is usually a supervisor. The results can be highly biased but it may not be. It entirely depends on the evaluator.
I personally think that checklists and rating scales are the best appraisal methods. This way, the measurement scale is the same for all employees and it is easier to analyze how the employee is improving by comparing future appraisals.
Also, regardless of which appraisal method is used, it means little if the employee does not receive feedback about his performance and what he can do to improve it.