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Title: Stacking for Transfer Learning

Paper content:
In machine learning tasks, overtting frequently crops up when the number of samples of target domain is insufﬁcient, for the generalization ability of the classiﬁer is poor in this circumstance.
To solve this problem, transfer learning utilizes the knowledge of similar domains to improve the robustness of the learner.
The main idea of existing transfer learning algorithms is to reduce the dierence between domains by sample selection or domain adaptation.
However, no matter what transfer learning algorithm we use, the difference always exists and the hybrid training of source and target data leads to reducing ﬁtting capability of the learner on target domain.
Moreover, when the relatedness between domains is too low, negative transfer is more likely to occur.
To tackle the problem, we proposed a two-phase transfer learning architecture based on ensemble learning, which uses the existing transfer learning algorithms to train the weak learners in the ﬁrst stage, and uses the predictions of target data to train the ﬁnal learner in the second stage.
Under this architecture, the ﬁtting capability and generalization capability can be guaranteed at the same time.
We evaluated the proposed method on public datasets, which demonstrates the effectiveness and robustness of our proposed method.
Transfer learning has attracted more and more attention since it was first proposed in 1995 BID11 and is becoming an important field of machine learning.
The main purpose of transfer learning is to solve the problem that the same distributed data is hard to get in practical applications by using different distributed data of similar domains.
Several different kinds of transfer stratagies are proposed in recent years, transfer learning can be devided into 4 categories BID17 , including instance-based transfer learning, feature-based transfer learning, parameter-based transfer learning and relation-based transfer learning.
In this paper, we focus on how to enhance the performance of instance-based transfer learning and feature-based transfer learning when limited labeled data from target domain can be obtained.
In transfer learning tasks, when diff-distribution data is obtained to improve the generalization ability of learners, the fitting ability on target data set will be affected more or less, especially when the domains are not relative enough, negative transfer might occur BID11 , it's hard to trade off between generalization and fitting.
Most of the existing methods to prevent negative transfer learning are based on similarity measure(e.g., maximum mean distance(MMD), KL divergence), which is used for choosing useful knowledge on source domains.
However, similarity and transferability are not equivalent concepts.
To solve those problems, we proposed a novel transfer learning architecture to improve the fitting capability of final learner on target domain and the generalization capability is provided by weak learners.
As shown in FIG0 , to decrease the learning error on target training set when limited labeled data on target domain can be obtained, ensemble learning is introduced and the performances of transfer learning algorithms are significantly improved as a result.In the first stage, traditional transfer learning algorithms are applied to diversify training data(e.g., Adaptive weight adjustment of boosting-based transfer learning or different parameter settings of domain adaptation).
Then diversified training data is fed to several weak classifiers to improve the generalization ability on target data.
To guarantee the fitting capability on target data, the predictions of target data is vectorized to be fed to the final estimator.
This architecture brings the following advantages:• When the similarity between domains is low, the final estimator can still achieve good performance on target training set.
Firstly, source data and target data are hybrid together to train the weak learners, then super learner is used to fit the predictions of target data.•
Parameter setting is simplified and performance is better than individual estimators under normal conditions.To test the effectiveness of the method, we respectively modified TrAdaboost BID1 and BDA BID16 as the base algorithms for data diversification and desired result is achieved.1.1 RELATED WORK
In this paper, we proposed a 2-phase transfer learning architecture, which uses the traditional transfer learning algorithm to achieve data diversification in the first stage and the target data is fitted in the second stage by stacking method, so the generalization ability and fitting ability on target data could be satisfied at the same time.
The experiment of instance-based transfer learning and feature-based transfer learning on 11 domains proves the validity of our method.
In summary, this framework has the following advantages:• No matter if source domain and target domain are similar, the training error on target data set can be minimized theoretically.•
We reduce the risk of negative transfer in a simple and effective way without a similarity measure.•
Introduction of ensemble learning gives a better performance than any single learner.•
Most existing transfer learning algorithm can be integrated into this framework.Moreover, there're still some problems require our further study, some other data diversification method for transfer learning might be useful in our model, such as changing the parameter µ in BDA, integrating multiple kinds of transfer learning algorithms, or even applying this framework for multi-source transfer learning.

assistant: text: How to use stacked generalization to improve the performance of existing transfer learning algorithms when limited labeled data is available.