If you've ever made anything from raw materials, you may have heard of that old but wise expression. It can apply equally well to process improvement as a reminder to measure the current state of your process carefully before making any changes. Whilst anecdotal evidence can be useful, hard facts are better. What you measure should align with what you are trying to improve, which in turn should link back to your customer. However, any changes can have unexpected results, so you should also look at your specific process as part of a larger system, and include some additional measures.
For example, making improvement to a single process is fine, so long as it doesn't have an adverse effect on another downstream process or maybe an external supplier or even a customer. Remember them? If you're focussing on variation or defects in a single process, then this bigger picture is easy to accidently overlook.