Multi-task learning: Difference between revisions

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{{short description|Solving multiple machine learning tasks at the same time}}

'''Multi-task learning''' (MTL) is a subfield of [[machine learning]] in which multiple learning tasks are solved at the same time, while exploiting commonalities and differences across tasks. This can result in improved learning efficiency and prediction accuracy for the task-specific models, when compared to training the models separately.<ref>Baxter, J. (2000). A model of inductive bias learning" ''Journal of Artificial Intelligence Research'' 12:149--198, [http://www-2.cs.cmu.edu/afs/cs/project/jair/pub/volume12/baxter00a.pdf On-line paper]</ref><ref>[[Sebastian Thrun|Thrun, S.]] (1996). Is learning the n-th thing any easier than learning the first?. In Advances in Neural Information Processing Systems 8, pp. 640--646. MIT Press. [http://citeseer.ist.psu.edu/thrun96is.html Paper at Citeseer]</ref><ref name=":2">{{Cite journal|url = http://www.cs.cornell.edu/~caruana/mlj97.pdf|title = Multi-task learning|last = Caruana|first = R.|date = 1997|journal = Machine Learning|doi = 10.1023/A:1007379606734|volume=28|pages=41–75|doi-access = free}}</ref> Early versions of MTL were called "hints".<ref>Suddarth, S., Kergosien, Y. (1990). Rule-injection hints as a means of improving network performance and learning time. EURASIP Workshop. Neural Networks pp. 120-129. Lecture Notes in Computer Science. Springer.</ref><ref>{{cite journal | last1 = Abu-Mostafa | first1 = Y. S. | year = 1990 | title = Learning from hints in neural networks | journal = Journal of Complexity | volume = 6 | issue = 2| pages = 192–198 | doi=10.1016/0885-064x(90)90006-y}}</ref>

In a widely cited 1997 paper, Rich Caruana gave the following characterization:<blockquote>Multitask Learning is an approach to [[inductive transfer]] that improves [[Generalization error|generalization]] by using the ___domain information contained in the training signals of related tasks as an [[inductive bias]]. It does this by learning tasks in parallel while using a shared [[Representation learning|representation]]; what is learned for each task can help other tasks be learned better.<ref name=":2">{{Cite journal|url = http://www.cs.cornell.edu/~caruana/mlj97.pdf|title = Multi-task learning|last = Caruana|first = R.|date = 1997|journal = Machine Learning|doi = 10.1023/A:1007379606734|volume=28|pages=41–75|doi-access = free}}</ref></blockquote>

In the classification context, MTL aims to improve the performance of multiple classification tasks by learning them jointly. One example is a spam-filter, which can be treated as distinct but related classification tasks across different users. To make this more concrete, consider that different people have different distributions of features which distinguish spam emails from legitimate ones, for example an English speaker may find that all emails in Russian are spam, not so for Russian speakers. Yet there is a definite commonality in this classification task across users, for example one common feature might be text related to money transfer. Solving each user's spam classification problem jointly via MTL can let the solutions inform each other and improve performance.<ref name=":0">{{Cite web|url = http://www.cs.cornell.edu/~kilian/research/multitasklearning/multitasklearning.html|title = Multi-task Learning|last = Weinberger|first = Kilian}}</ref> Further examples of settings for MTL include [[multiclass classification]] and [[multi-label classification]].<ref name=":1">{{Cite arxiv|eprint = 1504.03101|title = Convex Learning of Multiple Tasks and their Structure|last = Ciliberto|first = C.|date = 2015 |class = cs.LG}}</ref>