In our calibration lab we have a saying, dirt measures. This actually means a lot more than just cleaning dimensional gages. It speaks to the fact that sources of measurement uncertainty are everywhere. In metrology speak we call them “contributors” to measurement uncertainty. These contributors are everywhere. We try to minimize their impact on the measurements we perform, but in order to do that we must identify them and then compute their magnitude. For this, blog we will skip the math and focus on identifying sources of measurement uncertainty.