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Quantitative structure–activity relationship
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<p>Quantitative structure–activity relationship models (QSAR models) are <a href="/facts/Regression_analysis/n6z5Tf7K">regression</a> or classification models used in the chemical and biological sciences and engineering. Like other regression models, QSAR regression models relate a set of "predictor" variables (X) to the potency of the <a href="/facts/Response_variable/dINfzIF2">response variable</a> (Y), while classification QSAR models relate the predictor variables to a categorical value of the response variable. </p><p>In QSAR modeling, the predictors consist of physico-chemical properties or theoretical <a href="/facts/Molecular_descriptor/peFY9klS">molecular descriptors</a> of chemicals; the QSAR response-variable could be a <a href="/facts/Biological_activity/jJ14jyvc">biological activity</a> of the chemicals. QSAR models first summarize a supposed relationship between <a href="/facts/Chemical_structure/r5rT1Lj4">chemical structures</a> and <a href="/facts/Biological_activity/jJ14jyvc">biological activity</a> in a data-set of chemicals. Second, QSAR models <a href="/facts/Predictive_inference/XoTGh909">predict</a> the activities of new chemicals. </p><p>Related terms include <i>quantitative structure–property relationships</i> (<i>QSPR</i>) when a chemical property is modeled as the response variable. "Different properties or behaviors of chemical molecules have been investigated in the field of QSPR. Some examples are quantitative structure–reactivity relationships (QSRRs), quantitative structure–chromatography relationships (QSCRs) and, quantitative structure–toxicity relationships (QSTRs), quantitative structure–electrochemistry relationships (QSERs), and quantitative structure–<a href="/facts/Biodegradability/d1UW4dfR">biodegradability</a> relationships (QSBRs)." </p><p>As an example, biological activity can be expressed quantitatively as the concentration of a substance required to give a certain biological response. Additionally, when physicochemical properties or structures are expressed by numbers, one can find a mathematical relationship, or quantitative structure-activity relationship, between the two. The mathematical expression, if carefully validated, can then be used to predict the modeled response of other chemical structures. </p><p>A QSAR has the form of a <a href="/facts/Mathematical_model/CGhyuxgz">mathematical model</a>: </p> <ul><li>Activity = <i>f</i> (physiochemical properties and/or structural properties) + error</li></ul> <p>The error includes model error (<a href="/facts/Bias_of_an_estimator/oxIvEgmd">bias</a>) and observational variability, that is, the variability in observations even on a correct model. </p>