Making careful quantitative measurements are important if we want to claim that a particular physics theory explains something about the world around us. Measurements represent some physical quantity. For example, 2.1 meter represents a distance, 7 kilograms represents a mass and 9.3 seconds represents a time. Notice that each of these quantities has a number (9.3) and a unit (“seconds”). The number tells you the amount and the unit tells you the thing that you are talking about. Both the number and the unit are required to specify a measured quantity. Leaving either out makes your answer wrong! Pay attention.
There is a certain inherent inaccuracy or variation in any measurement we make in the laboratory. This inherent inaccuracy or variation is called experimental “error” and the word is not meant to imply incompetence on the part of the experimenter. Error merely reflects the condition that our measuring instruments are imperfect. This lack of perfection in our measuring procedure is to be contrasted with mistakes like