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Accuracy and precision
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<p>Accuracy and precision are two measures of <i><a href="/facts/Observational_error/q7pa7bRt">observational error</a></i>. <i>Accuracy</i> is how close a given set of <a href="/facts/Measurements/rsLLbn1s">measurements</a> (<a href="/facts/Observation/NTodmzCo">observations</a> or readings) are to their <i><a href="/facts/True_value/fkJju25M">true value</a></i>. <i>Precision</i> is how close the measurements are to each other. </p><p>The <a href="/facts/International_Organization_for_Standardization/IacYR7Yg">International Organization for Standardization</a> (ISO) defines a related measure: <i>trueness</i>, "the closeness of agreement between the <a href="/facts/Arithmetic_mean/L3Pv333o">arithmetic mean</a> of a large number of test results and the true or accepted reference value." </p> <p>While <i>precision</i> is a description of <i><a href="/facts/Random_errors/q7pa7bRt">random errors</a></i> (a measure of <a href="/facts/Statistical_variability/xqevRHmz">statistical variability</a>), <i>accuracy</i> has two different definitions: </p> <ol><li>More commonly, a description of <i><a href="/facts/Systematic_errors/q7pa7bRt">systematic errors</a></i> (a measure of <a href="/facts/Statistical_bias/Pql981K3">statistical bias</a> of a given measure of <a href="/facts/Central_tendency/9yhcQ6me">central tendency</a>, such as the mean). In this definition of "accuracy", the concept is independent of "precision", so a particular set of data can be said to be accurate, precise, both, or neither. This concept corresponds to ISO's <i>trueness</i>.</li> <li>A combination of both precision and trueness, accounting for the two types of observational error (random and systematic), so that high accuracy requires both high precision and high trueness. This usage corresponds to ISO's definition of <i>accuracy</i> (trueness and precision).</li></ol>