Editing Feature (machine learning)

{{Short description|Measurable property or characteristic}}
{{distinguish|Feature (computer vision)}}
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In [[machine learning]] and [[pattern recognition]], a '''feature''' is an individual measurable property or characteristic of a data set.<ref name="ml">{{cite book |author=Bishop, Christopher |title=Pattern recognition and machine learning |publisher=Springer |location=Berlin |year=2006 |isbn=0-387-31073-8 }}</ref> Choosing informative, discriminating, and independent features is crucial to produce effective [[algorithm]]s for [[pattern recognition]], [[Classification (machine learning)|classification]], and [[Regression analysis|regression]] tasks. Features are usually numeric, but other types such as [[String (computer science)|strings]] and [[Graph (discrete mathematics)|graphs]] are used in [[syntactic pattern recognition]], after some pre-processing step such as [[One-hot|one-hot encoding]]. The concept of "features" is related to that of [[Dependent and independent variables|explanatory variable]]s used in statistical techniques such as [[linear regression]].

==Feature types==
In feature engineering, two types of features are commonly used: numerical and categorical.

Numerical features are continuous values that can be measured on a scale. Examples of numerical features include age, height, weight, and income. Numerical features can be used in machine learning algorithms directly.{{cn|date=June 2024}}

[[Categorical variable|Categorical features]] are discrete values that can be grouped into categories. Examples of categorical features include gender, color, and zip code. Categorical features typically need to be converted to numerical features before they can be used in machine learning algorithms. This can be done using a variety of techniques, such as one-hot encoding, label encoding, and ordinal encoding.

The type of feature that is used in feature engineering depends on the specific machine learning algorithm that is being used. Some machine learning algorithms, such as decision trees, can handle both numerical and categorical features. Other machine learning algorithms, such as linear regression, can only handle numerical features.

==Classification==

A numeric feature can be conveniently described by a feature vector. One way to achieve [[binary classification]] is using a [[linear predictor function]] (related to the [[perceptron]]) with a feature vector as input. The method consists of calculating the [[Dot product|scalar product]] between the feature vector and a vector of weights, qualifying those observations whose result exceeds a threshold.
 
Algorithms for classification from a feature vector include [[k-nearest neighbor algorithm|nearest neighbor classification]], [[Artificial neural network|neural networks]], and [[statistical classification|statistical techniques]] such as [[Bayesian inference|Bayesian approaches]].

==Examples==
{{also|Feature (computer vision)}}
In [[character recognition]], features may include [[histogram]]s counting the number of black pixels along horizontal and vertical directions, number of internal holes, stroke detection and many others.

In [[speech recognition]], features for recognizing [[phonemes]] can include noise ratios, length of sounds, relative power, filter matches and many others.

In [[spam (electronic)|spam]] detection algorithms, features may include the presence or absence of certain email headers, 
the email structure, the language, the frequency of specific terms, the grammatical correctness of the text.

In [[computer vision]], there are a large number of possible [[feature (computer vision)|features]], such as edges and objects.

==Feature vectors==
{{See also|Word embedding}}
{{Redirect|Feature space|feature spaces in kernel machines|Kernel method}}
In [[pattern recognition]] and [[machine learning]], a '''feature vector''' is an n-dimensional [[Vector (mathematics and physics)|vector]] of numerical features that represent some object. Many [[algorithm]]s in machine learning require a numerical representation of objects, since such representations facilitate processing and statistical analysis. When representing images, the feature values might correspond to the pixels of an image, while when representing texts the features might be the frequencies of occurrence of textual terms. Feature vectors are equivalent to the vectors of [[explanatory variable]]s used in [[statistics|statistical]] procedures such as [[linear regression]].  Feature vectors are often combined with weights using a [[dot product]] in order to construct a [[linear predictor function]] that is used to determine a score for making a prediction.

The [[vector space]] associated with these vectors is often called the '''feature space'''. In order to reduce the dimensionality of the feature space, a number of [[dimensionality reduction]] techniques can be employed.

Higher-level features can be obtained from already available features and added to the feature vector; for example, for the study of diseases the feature 'Age' is useful and is defined as ''Age = 'Year of death' minus 'Year of birth' ''. This process is referred to as '''feature construction'''.<ref name=Liu1998>Liu, H., Motoda H. (1998) ''[https://books.google.com/books?id=aaDbBwAAQBAJ Feature Selection for Knowledge Discovery and Data Mining].'', Kluwer Academic Publishers. Norwell, MA, USA. 1998.</ref><ref name=Piramithu2009>Piramuthu, S., Sikora R. T. [https://www.sciencedirect.com/science/article/pii/S0957417408001309 Iterative feature construction for improving inductive learning algorithms]. In Journal of Expert Systems with Applications. Vol. 36 , Iss. 2 (March 2009), pp. 3401-3406, 2009</ref> Feature construction is the application of a set of constructive operators to a set of existing features resulting in construction of new features. Examples of such constructive operators include checking for the equality conditions {=, ≠}, the arithmetic operators {+,−,×, /}, the array operators {max(S), min(S), average(S)} as well as other more sophisticated operators, for example count(S,C)<ref name=bloedorn1998>Bloedorn, E., Michalski, R. Data-driven constructive induction: a methodology and its applications. IEEE Intelligent Systems, Special issue on Feature Transformation and Subset Selection, pp. 30-37, March/April, 1998</ref> that counts the number of features in the feature vector S satisfying some condition C or, for example, distances to other recognition classes generalized by some accepting device. Feature construction has long been considered a powerful tool for increasing both accuracy and understanding of structure, particularly in high-dimensional problems.<ref name=breinman1984>Breiman, L. Friedman, T., Olshen, R., Stone, C. (1984) ''Classification and regression trees'', Wadsworth</ref> Applications include studies of disease and [[emotion recognition]] from speech.<ref name=Sidorova2009>Sidorova, J., Badia T. [https://ieeexplore.ieee.org/abstract/document/5402574/ Syntactic learning for ESEDA.1, tool for enhanced speech emotion detection and analysis]. Internet Technology and Secured Transactions Conference 2009 (ICITST-2009), London, November 9–12. IEEE</ref>

==Selection and extraction==
{{main|Feature selection|Feature extraction}}

The initial set of raw features can be redundant and large enough that estimation and optimization is made difficult or ineffective. Therefore, a preliminary step in many applications of [[machine learning]] and [[pattern recognition]] consists of [[Feature selection|selecting]] a subset of features, or [[Feature extraction|constructing]] a new and reduced set of features to facilitate learning, and to improve generalization and interpretability.<ref>{{Cite book |last1=Hastie |first1=Trevor |url=https://books.google.com/books?id=eBSgoAEACAAJ |title=The Elements of Statistical Learning: Data Mining, Inference, and Prediction |last2=Tibshirani |first2=Robert |last3=Friedman |first3=Jerome H. |date=2009 |publisher=Springer |isbn=978-0-387-84884-6 |language=en}}</ref>

Extracting or selecting features is a combination of art and science; developing systems to do so is known as [[feature engineering]]. It requires the experimentation of multiple possibilities and the combination of automated techniques with the intuition and knowledge of the [[domain expert]]. Automating this process is [[feature learning]], where a machine not only uses features for learning, but learns the features itself.

== See also ==
* [[Covariate]]
* [[Dimensionality reduction]]
* [[Feature engineering]]
* [[Hashing trick]]
* [[Statistical classification]]
* [[Explainable artificial intelligence]]

==References==
{{Reflist}}

[[Category:Data mining]]
[[Category:Machine learning]]
[[Category:Pattern recognition]]