# A Generic Coordinate Descent Framework for Learning from Implicit Feedback

Proceedings of the 26th International Conference on World Wide Web, pp. 1341-1350, 2017.

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Abstract:

In recent years, interest in recommender research has shifted from explicit feedback towards implicit feedback data. A diversity of complex models has been proposed for a wide variety of applications. Despite this, learning from implicit feedback is still computationally challenging. So far, most work relies on stochastic gradient descent...More

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Introduction

- The focus of recommender system research has shifted from explicit feedback problems such as rating prediction to implicit feedback problems.
- Most of the signal that a user provides about her preferences is implicit.
- Examples for implicit feedback are: a user watches a video, clicks on a link, etc.
- Implicit feedback data is much cheaper to c 2017 International World Wide Web Conference Committee (IW3C2), published under Creative Commons CC-BY-NC-ND 2.0 License.
- WWW 2017, April 3–7, 2017, Perth, Australia.

Highlights

- The focus of recommender system research has shifted from explicit feedback problems such as rating prediction to implicit feedback problems
- In Section 5, we show how to apply iCD to a diverse set of models, including, matrix factorization (MF), factorization machines (FM) and tensor factorization
- In Section 5, we show that many common models are k-separable, including matrix factorization, feature-based approaches such as factorization machines, and higher-order tensor factorization such as Parallel Factor Analysis or Tucker decomposition
- We have presented a general, efficient framework for learning recommender system models from implicit feedback
- We have shown that the implicit regularizer of any k-separable model can be computed efficiently without iterating over all context-item pairs

Methods

- The main objective of the experiments is to illustrate the generality of the iCD framework.
- The purpose of the experiments is not to compare BPR and CD on yet another dataset, but rather to demonstrate the versatility of the iCD framework and illustrate how it can serve as a building block for future research on complex recommender models.
- As with MF, it is likely that both iCD and BPR will show strengths in different applications

Results

- 6.2.1 Cold-Start Recommendation

In the Cold-Start recommendation [2] scenario, the authors assume that a user interacts with the recommender system for the first time.

Conclusion

- The authors have presented a general, efficient framework for learning recommender system models from implicit feedback.
- The authors have shown that the implicit regularizer of any k-separable model can be computed efficiently without iterating over all context-item pairs.
- The authors have provided efficient learning algorithms for these models based on the framework.
- The authors' framework is not limited to the models discussed in the paper but designed to serve as a general blueprint for deriving learning algorithms for recommender systems

Summary

## Introduction:

The focus of recommender system research has shifted from explicit feedback problems such as rating prediction to implicit feedback problems.- Most of the signal that a user provides about her preferences is implicit.
- Examples for implicit feedback are: a user watches a video, clicks on a link, etc.
- Implicit feedback data is much cheaper to c 2017 International World Wide Web Conference Committee (IW3C2), published under Creative Commons CC-BY-NC-ND 2.0 License.
- WWW 2017, April 3–7, 2017, Perth, Australia.
## Methods:

The main objective of the experiments is to illustrate the generality of the iCD framework.- The purpose of the experiments is not to compare BPR and CD on yet another dataset, but rather to demonstrate the versatility of the iCD framework and illustrate how it can serve as a building block for future research on complex recommender models.
- As with MF, it is likely that both iCD and BPR will show strengths in different applications
## Results:

6.2.1 Cold-Start Recommendation

In the Cold-Start recommendation [2] scenario, the authors assume that a user interacts with the recommender system for the first time.## Conclusion:

The authors have presented a general, efficient framework for learning recommender system models from implicit feedback.- The authors have shown that the implicit regularizer of any k-separable model can be computed efficiently without iterating over all context-item pairs.
- The authors have provided efficient learning algorithms for these models based on the framework.
- The authors' framework is not limited to the models discussed in the paper but designed to serve as a general blueprint for deriving learning algorithms for recommender systems

Related work

- Since several years, matrix factorization (MF) is regarded as the most effective, basic recommender system model. Two optimization strategies dominate the research on MF from implicit feedback data. The first one is Bayesian Personalized Ranking (BPR) [13], a stochastic gradient descent (SGD) framework, that contrasts pairs of consumed to nonconsumed items. The second one is coordinate descent (CD) also known as alternating least squares on an elementwise loss over both the consumed and non-consumed items [5]. In terms of the loss formulation, BPR’s pairwise classification loss is better suited for ranking whereas CD loss is better suited for numerical data. With regard to the optimization task, both techniques face the same challenge of learning over a very large number of training examples. BPR tackles this issue by sampling negative items, but it has been shown that BPR has convergence problems when the number of items is large [7, 12]. It requires more complex, nonuniform, sampling strategies for dealing with this problem [12, 6]. On the other hand, for CD-MF, Hu et al [5] have derived an efficient algorithm that allows to optimize over the large number of non-consumed items without any cost. This computational trick is exact and does not involve sampling. Many authors have compared both CD-MF and BPR-MF on a variety of datasets and some work reports better quality for BPR-MF [4, 17, 16, 8] whereas for other problems CD-MF works better [8, 25, 15, 22, 26]. This large body of results indicates that the advantages of CD and BPR are orthogonal and both approaches have their merits.

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