Hi,
A 100% reduction in subjectivity is not possible, as people can misread, misinterpret, and be biased. However, the degree of subjectivity can be reduced in a normal 90-degree appraisal system through:
a) A review by a second-level manager
b) Normalization process by the final rating committee
Other methods include adopting better systems (which may become administratively unwieldy) like a 360-degree appraisal system. This system, by virtue of multiple inputs from various sources, can balance extremes and reinforce what most people largely believe about an individual's performance.
Furthermore, a better rewards and recognition system can help. The goal is to assess consistency in performance over time. There may have been periods of extraordinary spikes or dips in performance. Schemes like spot bonuses can reward these spikes, and during the final evaluation, they may be taken into account. Informing the normalization committee about these instances can significantly reduce subjectivity.
Regards