Multi-class classification

  • Multi-class classification is a type of classification task with more than two classes, for e.g., classifying a set of images of fruits which may be oranges, apples, or pears. Multi-class classification makes the assumption that each sample is assigned to one and only one label: a fruit can be either an apple or a pear but not both at the same time. In other words, multi-class classification assumes that the labels are mutually exclusive.
  • Some examples of multi-class classification problems are sentiment classification, spam detection, etc.

Multi-label classification

  • Multi-label classification is a type of classification task where each training example can have more than one labels. The task of multi-label classification can be thought of as predicting the properties of a data-point that are not mutually exclusive, such as topics that are relevant for a document. A text might be about any of religion, politics, finance or education at the same time or none of these.

Examples

  • A multi-class problem has the assignment of instances to one of a finite, mutually-exclusive collection of classes. As in the example already given of crabs (from @Dikran): male-blue, female-blue, male-orange, female-orange. Each of these is exclusive of the others and taken together they are comprehensive.
  • One form of a multi-label problem is to divide these into two labels, sex and color; where sex can be male or female, and color can be blue or orange. But note that this is a special case of the multi-label problem as every instance will get every label (that is every crab has both a sex and a color).
  • Multi-label problems also include other cases that allow for a variable number of labels to be assigned to each instance. For instance, an article in a newspaper or wire service may be assigned to the categories NEWS, POLITICS, SPORTS, MEDICINE, etc. One story about an important sporting event would get an assignment of the label SPORTS; while another, involving political tensions that are revealed by a particular sporting event, might get both the labels SPORTS and POLITICS. Where I am, in the US, the results of the Superbowl are labeled both SPORTS and NEWS given the societal impact of the event.
  • Note that this form of labeling, with variable numbers of labels, can be recast into a form similar to the example with the crabs; except that every label is treated as LABEL-X or not-LABEL-X. But not all methods require this recasting.
  • Multi Class classification Problem One right answer and Mutually exclusive outputs(eg iris, numbers) Multi Label Classification more than one right answer and appropriate output or Non exclusive eg(sugar test, eye test)

Graphical interpretation

  • To complement the other answers, here are some figures. Each row represents the expected output for one sample.

Multi-class classification

  • One column = one class (one-hot encoding).
  • In the Multi-class case, there are more than 2 classes in total.

Multi-label classification

  • One column = one class,
  • In the Multi-label case, one sample might be assigned more than one class.

Multioutput-Multi-class classification

  • As a side note, nothing prevents you from having a multioutput-Multi-class classification problem, e.g.:

Multi-class vs. Multi-label Classification

References

Citation

If you found our work useful, please cite it as:

@article{Chadha2020DistilledMulti-classvsMulti-labelClassification,
  title   = {Multi-class vs. Multi-label Classification},
  author  = {Chadha, Aman},
  journal = {Distilled AI},
  year    = {2020},
  note    = {\url{https://aman.ai}}
}