• Precision : Precision describes the reproducibility of result that is, the agreement between numerical values for two or more replicate measurements, or measurement that have been made in exactly the same way. Generally, the precision of an analytical method is readily obtained by simply repeating the measurement.

Three terms are widely used to describe the precision of a set of replicate data : standard deviation, variance, and coefficient of variation. 

• Accuracy : Accuracy describes the correctness of an experimental result. Strictly speaking, the only type of measurement that can be complete accurate is one that involves counting objects. All other measurement contain errors give only an approximation of the truth.

Accuracy is a relative term in the sense that what is an accurate or inaccurate method very much depends upon the needs of the scientist and the difficulty of the analytical problem. For example, an analytical method the yields results that are within +/-10 %, or one part per billion , of the correct amount of mercury in a sample of fish tissue that contains 10 parts per billion of the metal would usually be considered to be reasonably accurate. In contrast, a procedure that yields results that are within +/- 10% of the correct amount of mercury in an ore that contains 20% of the metal would usually be deemed unacceptably inaccurate.

Accuracy of the measurement depends on the method and the calibration while precision primarily reflects the quality of the instrumentation and data handling. 

2. Systematic errors :

Systematic errors have three sources : instrumental, personal, and method.

• Instrument errors : Typical sources of instrumental errors include drift in electronic circuits, leakage in vacuum system, temperature effects on detectors, currents induced in circuits from ac power lines, decreases in voltages of batteries with use, and calibration errors in meters, weights, and volumetric equipment.

Systematic instrument errors are commonly detected and corrected by calibration with suitable standards. Periodic calibration of instrument is always desirable with time as a consequence of wear, corrosion, or mistreatment.

• Personal errors : Personal errors are those introduced into a measurement by judgments that the experimentalist must make. Examples include estimating the position of a pointer between two scale divisions, the color of a solution at the end point in a titration, the level of a liquid with respect to a graduation in a pipet, or the relative intensity of two light bean. Judgments of this type are often subject to systematic, unidirectional uncertainties. For example, one person may read a pointer consistently high , another may be slightly slow in activating a timer, and a third may be less sensitive to color. Color blindness or other physical handicaps often exacerbate determinate personal errors.

Most personal errors can be minimized by care and self-discipline. Thus, most scientists develop the habit of systematically double-checking instrument readings, notebook entries, and calculations, Robots, automated systems, computerized data collection, and computerized instrument control have the potential of minimizing of eliminating personal systematic errors. 

• Method Errors. Method-based errors are often introduced from nonideal chemical and physical behavior of reagent and reactions upon which an analysis is based. Possible source include slowness or incompleteness of chemical reactions, losses by volatility, adsorption of the analyte on solids, instability of reagents, contaminants, and chemical interferences.

Systematic method errors are usually more difficult to detect and correct than are instrument and personal errors. The best and surest way involves validation of the method by employing it for the analysis of standard materials that resemble the samples to be analyzed both in physical state and in chemical composition. The analyte concentrations of these standards must, of course, be known with a high degree of certainty. For simple materials, standards can sometimes be prepared by blending carefully measured amounts of pure compounds. Unfortunately, more often than not, materials to be analyzed are sufficiently complex to preclude this simple approach.