The term precision refers to the number of decimal places of all the numbers used in the calculation. The precision of a measurement is determined by the instrument used and the skill of the person using it. In a normal lab, we would use instruments of different precision. In each case, you should get the most precision (significant figures) possible from each instrument you use. The smallest marked division on an instrument is called the least count. Whenever possible, you should read or estimate between the lines of the smallest divisions. This extra digit is called the fractional part. Accuracy refers to how close a measurement matches the actual value of that quantity. A measuring instrument must be correctly calibrated and properly used to yield measurements of high accuracy. In general, the accuracy of a measurement can be improved by making several measurements and calculating the average. The range of values that encompasses the calculated average can be used to express the uncertainty of the measurement. Suppose the following three measurements were taken (24.3 cm, 23.8 cm, and 24.8 cm). The result could be expressed as the average value in cm. Uncertainty: The level of confidence in the accuracy of a measurement can be expressed in terms of the uncertainty of the measurement. The uncertainty of measurement is expressed as a range of values that the experimenter believes includes the actual or true value of the measured quantity. For example, a measurement of 16.4 +- 0.5 cm (centimeters) has an uncertainty of +/- 0.5 cm. The actual or true value is expected to lie between 15.0 cm and 16.9 cm.