The number associated with a measurement is obtained using some measuring device. Consider the measurement of the volume of a liquid using a burette. Notice that the meniscus of the liquid occurs about 20.15 mL. This means that about 20.15 mL of a liquid has been delivered from the burette if the initial volume was 0.00 mL. Note that we must estimate the last number of the volume reading by interpolating between the 0.1 mL-marks. Since the last number is estimated, its value may be different if another person makes the same reading. If 5 different people read the same volume , the results might be as follows: 20.15 mL, 20.14 mL, 20.16 mL, 20.17 mL and 20.16 mL. These results show that the first 3 numbers (20.1) remain the same regardless of who makes the measurement, these are called certain digits. However, the digit to the right of the 1 must be estimated and therefore varies, it is called uncertain digit. We report a measurement by recording all the certain digits plus the first uncertain digit. These numbers are called significant figures. The uncertainty is the last number is usually assumed to be +/-1 unless otherwise stated.
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