The accuracy is the comparison between measurement and the true or most probable value. The comparison is done with regard to the error i.e. the accuracy is inversely proportional to the error. Accuracy is a general concept and it expresses the correctness of a measurement. Only single measurement provide accurate result. Accuracy can be determined by absolute method & comparative method.
The precision is the concordance of a series of measurements of the same quantity. Precision is more of a mathematical concept. It expresses reproducibility and repeatability. To get precise result several measurements is required which provide precise answers. Precision can be measured by the standard deviation and the variance.
Precision always accompanies by accuracy. But high degree of precision does not imply accuracy.
Example: Let, a substance contain 49.10±0.02 % of constituent Ă˘â‚¬ËśAĂ˘â‚¬â„˘ & the result obtained by two analysts using the same substance and the same analytical method are as follows;
ANALYST-1:- 49.01, 49.25, 49.08, 49.14%
The average value is 49.12 and the result ranges from 49.01 to 49.25%
ANALYST-2:- 49.40, 49.42, 49.42, 49.44%
The average value is 49.42 and the result ranges from 49.40 to 49.44%
(a). the values obtained by analyst-1 are accurate (very close to the correct result), but the precision is inferior to the results given by analyst-2. The values obtained by analyst-2 are very precise but are not accurate.
(b). the results of analyst-1 occur on both sides of the mean value and could be attributed to the random errors. It is apparent that there is a constant (systematic error) error present in the results of analyst-2.
(c). if analyst 2 had made the determinations on the same day in rapid succession, then this would be defined as Ă˘â‚¬ËśrepeatableĂ˘â‚¬â„˘ analysis. However if the determinations had been made on separate days when laboratory conditions may vary, this set of results would be defined as Ă˘â‚¬ËśreproducibleĂ˘â‚¬â„˘.
so, the lesser the scatter, higher the precision.
A measurement system can be accurate but not precise, precise but not accurate, neither, or both. For example, if an experiment contains a systematic error, then increasing the sample size generally increases precision but does not improve accuracy. Eliminating the systematic error improves accuracy but does not change precision. A measurement system is called valid if it is both accurate and precise.
Repeatability Ă˘â‚¬â€ť the variation arising when all efforts are made to keep conditions constant by using the same instrument and operator, and repeating during a short time period.
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