{"id":38,"date":"2024-04-04T00:39:47","date_gmt":"2024-04-03T21:39:47","guid":{"rendered":"https:\/\/sisu.ut.ee\/lcms_method_validation\/42-calc_precision\/"},"modified":"2024-04-04T00:41:46","modified_gmt":"2024-04-03T21:41:46","slug":"42-calc_precision","status":"publish","type":"page","link":"https:\/\/sisu.ut.ee\/lcms_method_validation\/42-calc_precision\/","title":{"rendered":"4.2. Calculation of precision"},"content":{"rendered":"

\n\t\u00a0\n<\/p>\n

\n\tEvaluation of precision requires a sufficient number of replicate measurements to be made on suitable materials. The materials should be representative of the test samples in terms of matrix and analyte concentration, homogeneity and stability, but do not need to be Certified Reference Materials (CRMs). The replicates should be independent, i.e. the entire measurement process, including any sample preparation steps, should be repeated. The minimum number of replicates varies in different validation guidelines, but is typically between 6 and 15 for each of the material used in the study.\n<\/p>\n

\n\tPrecision is usually expressed as \u201cimprecision\u201d: absolute standard deviation (s<\/em>) (Eq 1), relative standard deviation (RSD) (Eq 2), variance (s<\/em>2<\/sup>) or coefficient of variation (CV) (same as RSD). Importantly, precision is not related to the\u00a0reference values.\u00a0\n<\/p>\n

\n\t\u00a0\"4-2-valem1b.png\"(Eq 1)<\/span>
\n<\/h6>\n
\n\t\u00a0\"4-2-valem2.png\"\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0(Eq 2)
\n<\/h6>\n

\n\ts<\/em> \u2013 standard deviation\n<\/p>\n

\n\tn<\/em> \u2013 number of measurements made\n<\/p>\n

\n\tx<\/em>i<\/sub>\u00a0\u2013 each measurement result\n<\/p>\n

\n\tx<\/em>mean<\/sub>\u00a0\u2013 mean value of the measurement results.\n<\/p>\n

\n\tIt is not possible to estimate a reliable standard deviation from data sets with few replicate measurements. If acceptable, the values calculated from several small sets of replicate measurements can be combined (pooled) in order to obtain estimates with sufficient number of degrees of freedom. The pooled standard deviation is calculated with Eq 3 or with Eq 4, if the number of measurements made with each sample is the same. Pooled standard deviation is useful if for some reason it is impossible to make a sufficient number of replicates with one sample \u2013 either because of the time constraints (e.g. stability) or because the available amount of sample is insufficient for replicates. Standard deviations can also be pooled when the same analyte is determined in different samples. However, in this case the samples should be reasonably similar (but not necessarily identical) in terms of the analyte content and matrix.\n<\/p>\n

\n\t\u00a0\"4-2-valem3b.png\"\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u00a0(Eq 3)<\/span><\/span>
\n<\/h6>\n
\n\t\"image006.png\"\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0(Eq 4)
\n<\/h6>\n

\n\tk<\/em> \u2013 number of samples\n<\/p>\n

\n\ts<\/em>1<\/span><\/sub>, s<\/em>2<\/span><\/sub>\u00a0etc \u2013 sample standard deviations\n<\/p>\n

\n\tn<\/em>1<\/span><\/sub>, n<\/em>2<\/sub>\u00a0<\/span>etc \u2013 number of measurements made for different samples\n<\/p>\n

\n\tGeneral formula for the case when the experiment is done with different samples, each measured on a single day:\n<\/p>\n

\n\t\"image001.png\"\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 (Eq 5)
\n<\/h6>\n

\n\tk \u2013<\/em>\u00a0number of groups (in this case samples)\n<\/p>\n

\n\tRSD<\/em>1<\/sub>, RSD<\/em>2<\/sub>, etc \u2013\u00a0within group standard deviations\n<\/p>\n

\n\tn<\/em>1<\/sub>, n<\/em>2<\/sub>, etc \u2013\u00a0numbers of measurements made for different samples\n<\/p>\n

\n\tPooled standard deviation is a weighted root mean square (RMS) average of standard deviations, whereby numbers of degrees of freedom of the individual standard deviations serve as weights.<\/p>\n","protected":false},"excerpt":{"rendered":"

\u00a0 Evaluation of precision requires a sufficient number of replicate measurements to be made on suitable materials. The materials should be representative of the test samples in terms of matrix and analyte concentration, homogeneity and stability, but do not need …<\/p>\n","protected":false},"author":60,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"class_list":["post-38","page","type-page","status-publish","hentry"],"acf":[],"_links":{"self":[{"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/pages\/38","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/users\/60"}],"replies":[{"embeddable":true,"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/comments?post=38"}],"version-history":[{"count":1,"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/pages\/38\/revisions"}],"predecessor-version":[{"id":846,"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/pages\/38\/revisions\/846"}],"wp:attachment":[{"href":"https:\/\/sisu.ut.ee\/lcms_method_validation\/wp-json\/wp\/v2\/media?parent=38"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}