Data challenges and the use of informal feedback
Everyone deserves a fair performance evaluation, right? Well, you're right, delivering a fair evaluation is your goal. But you do need to be aware of the limits of different types of data. You'll often hear people talk about the need for objective data. But the truth is that objective data about a person's performance at work is next to impossible to gather. There are exceptions. Think about sales, for example. It's easy to know how much someone did or did not sell. Similarly, for an employee who works in a manufacturing environment, you can get data on units produced, scrap, or other measures of quality or productivity.
However, when you're looking at office workers, measurement becomes far more difficult. We can and do use many types of ratings. And while useful, they're better described as subjective, not objective. Even in cases like sales or manufacturing, the objective data only covers half of what you wish to measure, which is the specific work being done. All of the things on the people side of the equation, things such.
Source: https://www.linkedin.com/learning/performance-review-foundations/data-challenges-and-the-use-of-informal-feedback