Editing Loss function (section)

{{Short description|Mathematical relation assigning a probability event to a cost}}
In [[mathematical optimization]] and [[decision theory]], a '''loss function''' or '''cost function''' (sometimes also called an error function)<ref name="ttf2001">{{cite book|first1=Trevor |last1=Hastie |authorlink1= |first2=Robert |last2=Tibshirani |authorlink2=Robert Tibshirani|first3=Jerome H. |last3=Friedman |authorlink3=Jerome H. Friedman |title=The Elements of Statistical Learning |publisher=Springer |year=2001 |isbn=0-387-95284-5 |page=18 |url=https://web.stanford.edu/~hastie/ElemStatLearn/}}</ref> is a function that maps an [[event (probability theory)|event]] or values of one or more variables onto a [[real number]] intuitively representing some "cost" associated with the event. An [[optimization problem]] seeks to minimize a loss function. An '''objective function''' is either a loss function or its opposite (in specific domains, variously called a [[reward function]], a [[profit function]], a [[utility function]], a [[fitness function]], etc.), in which case it is to be maximized. The loss function could include terms from several levels of the hierarchy.

In statistics, typically a  loss function is used for [[parameter estimation]], and the event in question is some function of the difference between estimated and true values for an instance of data. The concept, as old as [[Pierre-Simon Laplace|Laplace]], was reintroduced in statistics by [[Abraham Wald]] in the middle of the 20th century.<ref>{{cite journal |first=A. |last=Wald |title=Statistical Decision Functions |journal=Apa Psycnet |publisher=Wiley |year=1950 |url=https://psycnet.apa.org/record/1951-01400-000}}</ref>  In the context of [[economics]], for example, this is usually [[economic cost]] or [[Regret (decision theory)|regret]].  In [[Statistical classification|classification]], it is the penalty for an incorrect classification of an example. In [[actuarial science]], it is used in an insurance context to model benefits paid over premiums, particularly since the works of [[Harald Cramér]] in the 1920s.<ref>{{cite book |last=Cramér |first=H. |year=1930 |title=On the mathematical theory of risk |publisher=Centraltryckeriet }}</ref> In [[optimal control]], the loss is the penalty for failing to achieve a desired value. In [[financial risk management]], the function is mapped to a monetary loss.
[[File:Comparison of loss functions.png|thumb|Comparison of common loss functions ([[Mean absolute error|MAE]], [[Symmetric mean absolute percentage error|SMAE]], [[Huber loss]], and Log-Cosh Loss)  used for regression]]