Transfer Learning. Pei-Hao (Eddy) Su 1 and Yingzhen Li 2. January 29, Outline Motivation Historical points Definition Case studies

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1 Transfer Learning Pei-Hao (Eddy) Su 1 and Yingzhen Li 2 1 Dialogue Systems Group and 2 Machine Learning Group January 29, 2015 Transfer Learning 1 / 41

2 Outline 1 Motivation 2 Historical points 3 Definition 4 Case studies Transfer Learning 2 / 41

3 Outline 1 Motivation 2 Historical points 3 Definition 4 Case studies Transfer Learning 3 / 41

4 Standard Supervised Learning Task Transfer Learning 4 / 41

5 Standard Supervised Learning Task Most ML tasks assume the training/test data are drawn from the same data space and the same distribution Transfer Learning 4 / 41

6 NLP tasks: POS, NER, Category labelling Modified from Gao et al. s presentation in KDD 08 Transfer Learning 5 / 41

7 Combine and get better result Modified from Gao et al. s presentation in KDD 08 Transfer Learning 6 / 41

8 Motivation Traditional ML tasks assume the training/test data are drawn from the same data space and the same distribution Insufficient labelled data result in poor prediction performance Lots of (un-)related existing data from various sources Start from scratch is always time-consuming Transfer knowledge from other sources may help! Transfer Learning 7 / 41

9 Motivation (Taylor et.al JMLR 09) Transfer Learning 8 / 41

10 Outline 1 Motivation 2 Historical points 3 Definition 4 Case studies Transfer Learning 9 / 41

11 Psychology and Education In 1901, Thorndike and Woodworth explored how individuals transfer similar characteristics shared by different contexts Transfer Learning 10 / 41

12 Psychology and Education In 1901, Thorndike and Woodworth explored how individuals transfer similar characteristics shared by different contexts In 1992, Perkins and Salomon published Transfer of Learning which defined different types of transfer Transfer Learning 10 / 41

13 Psychology and Education In 1901, Thorndike and Woodworth explored how individuals transfer similar characteristics shared by different contexts In 1992, Perkins and Salomon published Transfer of Learning which defined different types of transfer examples: Skill learning: C/C + + Python Language acquisition: German English Transfer Learning 10 / 41

14 Machine Learning Transfer Learning 11 / 41

15 Machine Learning Explanation-Based Neural Network Learning: A Lifelong Learning Approach [Thrun PhD 95, NIPS 96] Transfer Learning 12 / 41

16 Machine Learning Explanation-Based Neural Network Learning: A Lifelong Learning Approach [Thrun PhD 95, NIPS 96] Multitask Learning [Caruana ICML 93 & 96, PhD 97] Transfer Learning 12 / 41

17 Machine Learning Explanation-Based Neural Network Learning: A Lifelong Learning Approach [Thrun PhD 95, NIPS 96] Multitask Learning [Caruana ICML 93 & 96, PhD 97] Workshops Learning to Learn: Knowledge Consolidation and Transfer in Inductive Systems [NIPS 95] Inductive Transfer: 10 Years Later [NIPS 05] Structural Knowledge Transfer for Machine Learning [ICML 06] Transfer Learning for Complex Tasks [AAAI 08] Lifelong Learning [AAAI 11] Theoretically Grounded Transfer Learning [ICML 13] Workshop: Second Workshop on Transfer and Multi-Task Learning: Theory meets Practice [NIPS 14]... Transfer Learning 12 / 41

18 Outline 1 Motivation 2 Historical points 3 Definition 4 Case studies Transfer Learning 13 / 41

19 Definition Notations Domain D 1 Data space X 2 Marginal distribution P(X ), where X X Task T (Given D = {X, P(X )}) 1 Label space Y 2 Learn a f : X Y to approach the underlying P(Y X ), where X X and Y Y Transfer Learning 14 / 41

20 Definition Assume we have only one source S and one target T : Definition Transfer Learning (TL): Given a source domain D S and learning task T S, a target domain D T and learning task T T, transfer learning aims to help improve the learning of the target predictive function f T ( ) in D T using the knowledge in D S and T S, where D S D T (either X S X T or P S (X ) P T (X )) or T S T T (either Y S Y T or P(Y S X S ) P(Y T X T )) Transfer Learning 15 / 41

21 Example: Category labelling Transfer Learning 16 / 41

22 Example: Category labelling Transfer Learning 17 / 41

23 Example: Category labelling Transfer Learning 18 / 41

24 ML v.s. TL (Langley 06, Yang et al. 13) Transfer Learning 19 / 41

25 Outline 1 Motivation 2 Historical points 3 Definition 4 Case studies Transfer Learning 20 / 41

26 Transfer in practice The rest of the talk will give you an intuition, with examples, on: when to transfer what to transfer and how to transfer Transfer Learning 21 / 41

27 When to transfer: Domain relatedness Transfer learning is applicable when there exists relatedness Standard machine learning assume source = target Transferring knowledge from unrelated domain can be harmful - Negative transfer [Rosenstein et al NIPS-05 Workshop] (Ben-David et al.) proposed a bound of target domain error Reference Ben-David et al. Analysis of Representation for Domain Adaptation. NIPS 06 Transfer Learning 22 / 41

28 When to transfer (Ben-David et al.) In standard binary classification supervised learning task: Given X, Y = {0, 1} and samples from P(x, y), we aim to learn f : X [0, 1] which captures P(y x) Often we decompose the problem into: 1 determine a feature mapping Φ : X Z 2 learn a hypothesis h : Z {0, 1} on dataset {Φ(x), y} In transfer learning scenario: Theorem (Simplified version of Thm. 1&2) Given X = X S = X T and P S (x), P T (x) the distributions of the source and target domain. Let Φ : X Z be a fixed mapping function and H be a hypothesis space. For any hypothesis h H trained on source domain: ɛ T (h) ɛ S (h) + d H( P S, P T ) + ɛ S (h ) + ɛ T (h ) where P S, P T are induced distributions on Z wrt. P S and P T, h = arg min h H (ɛ S (h) + ɛ T (h)) is the best hypothesis by joint training. Transfer Learning 23 / 41

29 Domain adaptation Approach 1: mixture of general & specific component Can we learn hypotheses for both the general and specific components? Reference: Daume III. Frustratingly easy domain adaptation. ACL 07 Daume III et al. Co-regularization Based Semi-supervised Domain Adaptation. NIPS 10 Transfer Learning 24 / 41

30 EasyAdapt (Daume III) Binary classification problem: X S = X T R d, Y S = Y T = { 1, +1} Goal: obtain classifier f T : X T Y T in SVM context: learn a hypothesis h T R d However: too little training data available on (X T, Y T ) for robust training also P(x S ) P(x T ) and P(x S, y T ) P(x S, y T )...so directly apply a trained hypothesis h s returns bad results How to use x S, y S P(x S, y S ) to improve learning of h T? Transfer Learning 25 / 41

31 EasyAdapt (Daume III) EasyAdapt algorithm define two mappings Φ S, Φ T : R d R 3d : Φ S (x S ) = (x S, x S, 0), Φ t(x T ) = (x T, 0, x T ) training: learn a hypothesis h = (w g, w s, w t ) R 3d on transformed dataset {(Φ S (x S ), y S )} {(Φ T (x T ), y T )} test: apply h T = w g + w t on x T (also h S = w g + w s ) Transfer Learning 26 / 41

32 EA++ (Daume III et al.) Use unlabelled data to improve training: want h S and h T to agree on unlabelled data x U : h S x U = h T x U w s x U = w t x U h (0, x U, x U ) = 0 so we define mapping Φ U : R d R 3d for unlabelled data Φ U (x U ) = (0, x U, x U ) (1) and train the hypothesis h on augmented and transformed dataset {(Φ S (x S ), y S )} {(Φ T (x T ), y T )} {(Φ U (x U ), 0)} Transfer Learning 27 / 41

33 EA++ (Daume III et al.) (a) DVD BOOKS (proxy A-distance=0.7616), (b) KITCHEN APPAREL (proxy A-distance=0.0459). SOURCE/TARGETONLY(-FULL): trained on source/target (full) labelled samples ALL: trained on combined labelled samples EA/EA++: trained in augmented feature space (and unlabelled target data) Transfer Learning 28 / 41

34 Feature transfer Approach 2: shared lower-level features DNN first layer learns Gabor filters or color blobs when trained on images instances in source/target domain share the same lower-level features? Reference: Yosinski et al. How transferable are features in deep neural networks? NIPS 14. Transfer Learning 29 / 41

35 Feature transfer 1 Lee et al. Convolutional deep belief networks for scalable unsupervised learning of hierarchical representations. ICML 09 1 adapt from Ruslan Salakhutdinov s tutorial in MLSS 14 Beijing Transfer Learning 30 / 41

36 Feature transfer (Yosinski et al.) Transfer Learning 31 / 41

37 Feature transfer (Yosinski et al.) Test 1 (similar datasets): random A/B splits of the ImageNet dataset (similar source and target domain training/testing instances) Transfer Learning 32 / 41

38 Feature transfer (Yosinski et al.) Test 1 (similar datasets): random A/B splits of the ImageNet dataset (similar source and target domain training/testing instances) Transfer Learning 32 / 41

39 Feature transfer (Yosinski et al.) Test 1 (similar datasets): random A/B splits of the ImageNet dataset (similar source and target domain training/testing instances) Transfer Learning 32 / 41

40 Feature transfer (Yosinski et al.) Test 2 (very different datasets): man-made/natural object split (dissimilar source and target domain training/testing instances) Transfer Learning 33 / 41

41 Feature transfer (Yosinski et al.) Test 2 (very different datasets): man-made/natural object split (dissimilar source and target domain training/testing instances) Transfer Learning 33 / 41

42 Joint representation Approach 3: joint feature representation data has many domain specific characteristics however might be related in high level? our brain might work like this as well Reference: Srivastava and Salakhutdinov. Multimodal Learning with Deep Boltzmann Machines. NIPS 12, JMLR 15 (2014). Transfer Learning 34 / 41

43 Joint representation (Srivastava et al.) MIR Flickr Dataset For images 1M datapoints, 25K labelled instances in 38 classes, 10K for training, 5K for validation and 10K for testing inputs are the concatenation of PHOW and MPEG-7 features For texts use word count vectors on 2K frequently used tags (very sparse) 18% training images have missing texts Transfer Learning 35 / 41

44 Joint representation (Srivastava et al.) for images: 2-layer deep Boltzmann machine (DBM) with Gaussian input units (v mi R, abbrev. W m (k) (i, j) as W (k) ij ) P(v m, h m (1), h m (2) ) exp (v mi b i ) 2 + 2σ 2 i i i,j v mi σ i W (1) ij h (1) mj + j,l h (1) mj W (2) jl h (2) ml Transfer Learning 36 / 41

45 Joint representation (Srivastava et al.) for texts: 2-layer DBM with replicated softmax model (v ti counts the occurrence of word i, abbrev. W (k) t (i, j) as W (k) P(v t, h (1) t, h (2) t ) exp v ti b i + i=1 i,j ij ) v ti W (1) ij h (1) mj + j,l h (1) tj W (2) jl h (2) tl Transfer Learning 36 / 41

46 Joint representation (Srivastava et al.) combining domain specific models to a multimodal DBM: P(v m, v t, h; θ) ( ) exp E(h m (2), h (2) t, h (3) ) E(v m, h m (1), h m (2) ) E(v t, h (1) t, h (2) t ) Transfer Learning 36 / 41

47 Joint representation (Srivastava et al.) first pre-train domain specific DBMs with CD, then co-train the joint model with PCD use mean-field variational approximation when computing hidden unit moments driven by data Transfer Learning 36 / 41

48 Joint representation (Srivastava et al.) Results: Figure: Classification with data from both image and text domain Figure: Classification with data from image domain only Transfer Learning 37 / 41

49 Joint representation (Srivastava et al.) Results: Figure: Retrieval results for multi/image domain queries Transfer Learning 37 / 41

50 Conclusions In this talk, we showed that transfer learning adapts knowledge from other sources to improve target task performance domains related to each other in different ways In the future: manage large scale data that do not lack in size but may lack in quality manage data which may continuously change over time Transfer Learning 38 / 41

51 Open Questions 2 what are the limits of existing multi-task learning methods when the number of tasks grows while each task is described by only a small bunch of samples ( big T, small n )? what is the right way to leverage over noisy data gathered from the Internet as reference for a new task? how can an automatic system process a continuous stream of information in time and progressively adapt for life-long learning? can deep learning help to learn the right representation (e.g., task similarity matrix) in kernel-based transfer and multi-task learning? How can similarities across languages help us adapt to different domains in natural language processing tasks?... 2 nips.cc/conferences/2014/program/event.php?id=4282 Transfer Learning 39 / 41

52 Thank you Transfer Learning 40 / 41

53 Reference 1 Pan and Yang. A Survey on Transfer Learning. IEEE TKDE Pan and Yang. Transfer Learning. MLSS Taylor et al. Transfer Learning for Reinforcement Learning Domains: A Survey. JMLR Langley. Transfer of Learning in Cognitive System. ICML Perkins et al. Transfer of Learning. IEE Thrun. Explanation-Based Neural Network Learning: A Lifelong Learning Approach. PhD thesis Caruana. Multitask Learning. PhD thesis Ben-David et al. Analysis of Representation for Domain Adaptation. NIPS Daume III. Frustratingly easy domain adaptation. ACL Daume III et al. Co-regularization Based Semi-supervised Domain Adaptation. NIPS Yosinski et al. How transferable are features in deep neural networks? NIPS Lee et al. Convolutional deep belief networks for scalable unsupervised learning of hierarchical representations. ICML 2009 Pei-Hao (Eddy) 13 Srivastava Su and Yingzhen and Li Salakhutdinov. Multimodal Learning with Deep Transfer Learning 41 / 41