View source for Binary classification

{{Short description|Classifying elements of a set into one or two groups}}
'''Binary classification''' is the task of [[Statistical classification|classifying]] the elements of a [[Set (mathematics)|set]] into one of two groups (each called ''class'') on the basis of a [[Classification rule|classification rule]]. Typical binary classification problems include:
* [[Medicine:Medical test|Medical test]]ing to determine if a patient has certain disease or not;
* [[Quality control]] in industry, deciding whether a specification has been met;
* In [[Information retrieval|information retrieval]], deciding whether a page should be in the [[Result set|result set]] of a search or not.

Binary classification is dichotomization applied to a practical situation. In many practical binary classification problems, the two groups are not symmetric, and rather than overall accuracy, the relative proportion of different [[Type I and type II errors|types of errors]] is of interest. For example, in medical testing, detecting a disease when it is not present (a ''[[False positives and false negatives#False positive error|false positive]]'') is considered differently from not detecting a disease when it is present (a ''[[False positives and false negatives#False negative error|false negative]]'').

==Statistical binary classification==
[[Statistical classification]] is a problem studied in [[Machine learning|machine learning]].  It is a type of [[Supervised learning|supervised learning]], a method of machine learning where the categories are predefined, and is used to categorize new probabilistic observations into said categories.  When there are only two categories the problem is known as statistical binary classification.

Some of the methods commonly used for binary classification are:
* [[Decision tree learning|Decision trees]]
* Random forests
* [[Bayesian network]]s
* [[Support vector machine]]s
* [[Neural network]]s
* [[Logistic regression]]
* [[Probit model]]
* Genetic Programming
* [[Multi expression programming]]
* [[Linear genetic programming]]

Each classifier is best in only a select domain based upon the number of observations, the dimensionality of the feature vector, the noise in the data and many other factors. For example, random forests perform better than [[Support vector machine|SVM]] classifiers for 3D point clouds.<ref>{{Cite journal|title = Automatic Identification of Window Regions on Indoor Point Clouds Using LiDAR and Cameras|last = Zhang & Zakhor|first = Richard & Avideh|date = 2014|journal = VIP Lab Publications|citeseerx = 10.1.1.649.303}}</ref><ref>{{Cite journal |title = Simplified markov random fields for efficient semantic labeling of 3D point clouds|last = Y. Lu and C. Rasmussen|date = 2012|journal = IROS|url=http://nameless.cis.udel.edu/pubs/2012/LR12/yan_iros2012.pdf}}</ref>

==Evaluation of binary classifiers==
{{main|Evaluation of binary classifiers}}

[[Image:binary-classification-labeled.svg|thumb|220px|right|In this set of tested instances, the instances left of the divider have the condition being tested; the right half do not. The oval bounds those instances that a test algorithm classifies as having the condition. The green areas highlight the instances that the test algorithm correctly classified. Labels refer to: <br />TP=true positive; TN=true negative; FP=false positive (type I error); FN=false negative (type II error); TPR=set of instances to determine true positive rate; FPR=set of instances to determine false positive rate; PPV=positive predictive value; NPV=negative predictive value.]]

There are many metrics that can be used to measure the performance of a classifier or predictor; different fields have different preferences for specific metrics due to different goals. In medicine [[Sensitivity and specificity|sensitivity and specificity]] are often used, while in information retrieval [[Precision and recall|precision and recall]] are preferred. An important distinction is between metrics that are independent of how often each category occurs in the population (the ''[[Prevalence|prevalence]]''), and metrics that depend on the prevalence – both types are useful, but they have very different properties.

Given a classification of a specific data set, there are four basic combinations of actual data category and assigned category: true positives TP (correct positive assignments), true negatives TN (correct negative assignments), false positives FP (incorrect positive assignments), and false negatives FN (incorrect negative assignments).

{| class="wikitable"
! {{diagonal split header|Actual|Assigned}}
! Test outcome ''positive''
! Test outcome ''negative''
|-
! Condition positive
| align="center"| True ''positive''
| align="center"| False ''negative''
|-
! Condition negative
| align="center"| False ''positive''
| align="center"| True ''negative''
|}
These can be arranged into a 2×2 [[Contingency table|contingency table]], with rows corresponding to actual value – condition positive or condition negative – and columns corresponding to classification value – test outcome positive or test outcome negative. 

===The eight basic ratios===
There are eight basic ratios that one can compute from this table, which come in four complementary pairs (each pair summing to 1). These are obtained by dividing each of the four numbers by the sum of its row or column, yielding eight numbers, which can be referred to generically in the form "true positive row ratio" or "false negative column ratio". 

There are thus two pairs of column ratios and two pairs of row ratios, and one can summarize these with four numbers by choosing one ratio from each pair – the other four numbers are the complements.

The row ratios are:
*true positive rate (TPR) = (TP/(TP+FN)), aka '''sensitivity''' or recall.  These are the proportion of the ''population with the condition'' for which the test is correct.
**with complement the false negative rate (FNR) = (FN/(TP+FN))
*true negative rate (TNR) = (TN/(TN+FP), aka '''specificity''' (SPC),
**with complement [[False positive rate|false positive rate]] (FPR) = (FP/(TN+FP)), also called independent of [[Prevalence|prevalence]]

The column ratios are:
*positive predictive value (PPV, aka precision) (TP/(TP+FP)).  These are the proportion of the ''population with a given test result'' for which the test is correct.
**with complement the [[False discovery rate|false discovery rate]] (FDR) (FP/(TP+FP))
*negative predictive value (NPV) (TN/(TN+FN))
**with complement the false omission rate (FOR) (FN/(TN+FN)), also called dependence on prevalence.

In diagnostic testing, the main ratios used are the true column ratios – true positive rate and true negative rate – where they are known as [[Sensitivity and specificity|sensitivity and specificity]]. In informational retrieval, the main ratios are the true positive ratios (row and column) – positive predictive value and true positive rate – where they are known as [[Precision and recall|precision and recall]]. There is no general theory that sets out which pair should be used in which circumstances; each discipline has its own reason for the choice it has made.

One can take ratios of a complementary pair of ratios, yielding four [[Medicine:Likelihood ratios in diagnostic testing|likelihood ratios]] (two column ratio of ratios, two row ratio of ratios). This is primarily done for the column (condition) ratios, yielding [[Medicine:Likelihood ratios in diagnostic testing|likelihood ratios in diagnostic testing]]. Taking the ratio of one of these groups of ratios yields a final ratio, the [[Diagnostic odds ratio|diagnostic odds ratio]] (DOR). This can also be defined directly as (TP×TN)/(FP×FN) = (TP/FN)/(FP/TN); this has a useful interpretation – as an [[Odds ratio|odds ratio]] – and is prevalence-independent.

There are a number of other metrics, most simply the [[Accuracy and precision#In binary classification|accuracy]] or Fraction Correct (FC), which measures the fraction of all instances that are correctly categorized; the complement is the Fraction Incorrect (FiC). The [[F-score]] combines precision and recall into one number via a choice of weighing, most simply equal weighing, as the balanced F-score ([[F1 score]]). Some metrics come from regression coefficients: the [[Social:Markedness|markedness]] and the informedness, and their [[Geometric mean|geometric mean]], the [[Biology:Matthews correlation coefficient|Matthews correlation coefficient]]. Other metrics include [[Youden's J statistic]], the [[Uncertainty coefficient|uncertainty coefficient]], the [[Biology:Phi coefficient|phi coefficient]], and [[Cohen's kappa]].

==Converting continuous values to binary==
{{anchor|artificial}} <!--Artificially binary value redirects here-->
Tests whose results are of continuous values, such as most blood values, can artificially be made binary by defining a cutoff value, with test results being designated as positive or negative depending on whether the resultant value is higher or lower than the cutoff.

However, such conversion causes a loss of information, as the resultant binary classification does not tell ''how much'' above or below the cutoff a value is. As a result, when converting a continuous value that is close to the cutoff to a binary one, the resultant positive or negative predictive value is generally higher than the predictive value given directly from the continuous value. In such cases, the designation of the test of being either positive or negative gives the appearance of an inappropriately high certainty, while the value is in fact in an interval of uncertainty. For example, with the urine concentration of [[Chemistry:Human chorionic gonadotropin|hCG]] as a continuous value, a urine [[Medicine:Pregnancy test|pregnancy test]] that measured 52 mIU/ml of hCG may show as "positive" with 50 mIU/ml as cutoff, but is in fact in an interval of uncertainty, which may be apparent only by knowing the original continuous value. On the other hand, a test result very far from the cutoff generally has a resultant positive or negative predictive value that is lower than the predictive value given from the continuous value. For example, a urine hCG value of 200,000 mIU/ml confers a very high probability of pregnancy, but conversion to binary values results in that it shows just as "positive" as the one of 52 mIU/ml.

==See also==
* [[Bayesian inference#Examples|Examples of Bayesian inference]]
* [[Classification rule]]
* [[Confusion matrix]]
* [[Detection theory]]
* Kernel methods
* [[Multiclass classification]]
* [[Multi-label classification]]
* [[One-class classification]]
* [[Prosecutor's fallacy]]
* [[Receiver operating characteristic]]
* [[Thresholding (image processing)]]
* [[Uncertainty coefficient]], aka proficiency
* [[Qualitative property]]
* [[Precision and recall]] (equivalent classification schema)

==References==
{{reflist}}

== Bibliography ==
* Nello Cristianini and [[Biography:John Shawe-Taylor|John Shawe-Taylor]]. ''An Introduction to Support Vector Machines and other kernel-based learning methods''. Cambridge University Press, 2000. {{ISBN|0-521-78019-5}} ''([https://web.archive.org/web/20180627015707/https://www.support-vector.net/] SVM Book)''
* John Shawe-Taylor and Nello Cristianini.  ''Kernel Methods for Pattern Analysis''.  Cambridge University Press, 2004.  {{ISBN|0-521-81397-2}} ([https://kernelmethods.blogs.bristol.ac.uk/ Website for the book])
* Bernhard Schölkopf and A. J. Smola: ''Learning with Kernels''. MIT Press, Cambridge, Massachusetts, 2002. {{ISBN|0-262-19475-9}}

{{Statistics|analysis||state=expanded}}

[[Category:Statistical classification]]
[[Category:Machine learning]]

{{Sourceattribution|Binary classification}}