By providing specific results, and accomplishments or examples you convey to the employee that your decision is based on facts. Thus, you use an employee's overall performance throughout the year to make your rating, rather than just the last couple of months. Cite specific examples of performance including the context in which it occurred, the behavior or inaction and the outcome or consequence. It is important to distinguish between fact and opinion in documenting performance. Documentation should focus on facts.
Facts are events, behaviors or results.
You are describing a fact when you describe what you know, what you take in from your five senses. Documentation should not include opinions, judgments or inferences. Opinions, judgments or inferences are subject to bias, and if they provide the sole basis for your ratings you will have little to justify the objectivity of your assessment.