Experimental work in any scientific field relies on measurements. Whether it is a geographic information system, a statistical test or an instrument in the laboratory, it is important for the analytical tool used in making these measurements to possess certain attributes. When researchers in any field use an analytical method, they need to prove that the method is capable of delivering results that are accurate as well as precise. Although this article uses the example of pharmaceutical testing, the concepts are equally relevant to all physical and social sciences.
Definition of Accuracy
Accuracy is sometimes also called “trueness,” and refers to how close a determined value is to the true value. The values obtained in a test can be compared with the known true values, provided the measurements have been performed under the prescribed conditions. This is a significant point, because a change in the experimental conditions will lead to a difference in the measurements. Replicated measurements are important to rule out instrumental and personal errors.
To measure the accuracy of a method in relation to a drug substance, the experimental value obtained is compared with the value of a reference standard whose purity is known. In case of drug products, the procedure adopted is to spike a placebo mixture with a known concentration of the active ingredient. The values determined by using the analytical method are then compared with this actual concentration to evaluate the accuracy of the method. At least three concentrations of the analyzed material are used, and determinations are repeated a minimum of five times for each concentration.
Definition of Precision
Precision refers to the closeness by which values are in agreement within a series of measurements of the same parameter. For example, if you measure the retention factor for an analyte by thin-layer chromatography, the precision of the method refers to the degree of scatter observed between each individual measurement of the retention-factor value. When determining the precision of a method, it is important to ensure you adhere to the prescribed experimental conditions, and carry out multiple sampling from the same homogenous sample, maintaining conditions exactly as the method prescribes.
Measuring the precision of an analytical method involves calculating the standard deviation between measurements under three categories. The first category consists of assessing the precision of a method under identical operating conditions within a short period of time within the same day. Intermediate precision is the next category, and is assessed by repeating determinations using the method on different days, by different analysts and with different instruments in the same laboratory. Reproducibility is the third category, and provides information about how precise the method is when it is used in different laboratories.