As a manager, it’s your job to make your employees’ performance reviews as objective and unbiased as possible. One way to make your reviews fairer is to ask for peer evaluations.
Instead of writing the assessment solely from your perspective (and perhaps asking the reviewee to do a self-assessment), ask your team members to write evaluations of one another.
They can then share their reviews with you and, if they’d like, with the person being evaluated. This will give you valuable input from the people who work closely with the reviewee every day. It will also help you temper any bias you might bring to the evaluation and encourage your team to be open and transparent with each other.