For all its ubiquity in the lab, proper QC isn't easy. Even though every laboratory has to perform Quality Control, that hasn't made the task any simpler. It still relies on the right mean, the right SD, the right control limits, the right rules and numbers of control measurements, and the right interpretation of control data points. If you get one part of this system wrong, it can throw off the correct implementation. Here's an example that was recently published showing the difficulties and challenges of performing proper QC.