1. Measurements are necessary for experiments, production, and quality control. Standard units are needed to make measurements reliable, accurate, and uniform for all. 2. The CGS, MKS, and SI systems define standard units for measurements of length, mass, and time. The SI system is now accepted internationally. 3. Accuracy refers to how close a measurement is to the true value, while precision refers to the consistency of repeated measurements. Errors are the differences between measured and actual values.