What is meant by accuracy and precision?
Accuracy
The term ‘accuracy’ refers to the agreement of experimental value with the true value and it is usually expressed in terms of error. Accuracy is also described as the degree of agreement between a measured value and the accepted true value. In scientific experiments since no measurement is completely accurate, the true value is not known within certain limits. It is simply taken as a value that has been accepted and is generally a mean calculated from the results of several determinations from many laboratories using different techniques.
Precision
Precision is defined as, “the degree of agreement between various results of the same quantity”. In other words, it is the reproducibility of the result. For example, if a result of an analysis is 6.18 when it was performed for the first time. If the analysis is repeated four times, and the values obtained are 6.17, 6.19, 6.18, and 6.17, then the precision is calculated by comparing the values with each other. The closeness of the values decides the precision of the method
Significant Figures
In analysis, it is important to understand the term ‘significant figures’. While recording the values measured in analysis, some errors do happen if the figures are not properly recorded. The number of significant figures can be defined as, “the number of digits necessary to express the results of a measurement consistent with the measured precision” Each digit denotes the actual quantity that it specifies. The proper manner of expressing a result or observations is to retain a number of figures that all are known with certainty except the last.
It should be clear that zeroes are employed to denote the significant part of measurement – to denote tens, hundreds, thousands, etc., or merely to locate the decimal point. Thus, zeroes within a number like 25.05 and 1350 are significant as they express the exact quantity while zeroes in figures like 0.0234 only show the magnitude of the other digits.
For example, in quantities 1.2670 g and 1.0056 g, the zeroes are significant, but in the quantity 0.0035 kg the zeroes are not as important; the zeroes in 0.0035 kg serve only to locate the decimal point; this is omissible by the use of proper units like 03.5 g. Thus, in 1.2570 g and 1.0056 g, there are five significant figures.
The digits of a number that are needed to express the precision of measurement must be retained. When a volume is between 15.7 ml and 15.9 ml should be written as 15.8 ml and not as 15.80; since the latter would indicate that the value is between 15.79 ml and 15.81 ml.
If the weight is to the nearest 0.1 mg, e.g. 3.280-0 g; it should not be written as 03.280 g or 03.28 g.
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