Expressing Precision with Significant Figures

The isotopic molar masses are precise to five or more significant figures, so we are tempted to express the result with five significant figures. The mass defect is determined by addition and subtraction, however, and two of the isotopic molar masses are known to just three decimal places, so the mass defect is precise to three decimal places, and the... 

They are very precise because they are within the uncertainty in the last significant figure expressed. It is not known with certainty if the results are accurate, although good precision usually indicates good accuracy. 

In performing pharmacokinetic calculations, we must take care to get the most precise answer that can be supported by the data we have. Conversely, we do not want to express our answer with greater precision than we are justified in claiming. The rules of significant figures will help us with this task. These are listed in Box 2.3. 

Note that the number of figures quoted in the preceding simple example has been truncated from that displayed on an electronic calculator, in accord with the rules for number of significant figures. Since experimental measurements with precision expressed as, e.g., or SE,j, are being dealt with here, it is obvious that numerical values of such parameters should not be quoted with... 

Precise measurements, however, are not necessarily accurate. The ACCURACY expresses the agreement of the measurement with the accepted value of the quantity. Accuracy is expressed in terms of the error, the experimentally determined value minus the accepted value. Significant figures are discussed in Appendix 1.14. 

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