Adversarial feature learning (AFL) is one of the promising ways for explicitly constrains neural networks to learn desired representations; for example, AFL could help to learn anonymized representations so as to avoid privacy issues. AFL learn such a representations by training the networks to deceive the adversary that predict the sensitive information from the network, and therefore, the success of the AFL heavily relies on the choice of the adversary. This paper proposes a novel design of the adversary, multiple adversaries over random subspaces (MARS) that instantiate the concept of the volunerableness. The proposed method is motivated by an assumption that deceiving an adversary could fail to give meaningful information if the adversary is easily fooled, and adversary rely on single classifier suffer from this issues. In contrast, the proposed method is designed to be less vulnerable, by utilizing the ensemble of independent classifiers where each classifier tries to predict sensitive variables from a different subset of the representations. The empirical validations on three user-anonymization tasks show that our proposed method achieves state-of-the-art performances in all three datasets without significantly harming the utility of data. This is significant because it gives new implications about designing the adversary, which is important to improve the performance of AFL.
The below review addresses the first revision of the paper. The revised version does address my concerns. The fact that the paper does not come with substantial theoretical contributions/justification still stands out.
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The authors present a variant of the adversarial feature learning (AFL) approach by Edwards & Storkey. AFL aims to find a data representation that allows to construct a predictive model for target variable Y, and at the same time prevents to build a predictor for sensitive variable S. The key idea is to solve a minimax problem where the log-likelihood of a model predicting Y is maximized, and the log-likelihood of an adversarial model predicting S is minimized. The authors suggest the use of multiple adversarial models, which can be interpreted as using an ensemble model instead of a single model.
The way the log-likelihoods of the multiple adversarial models are aggregated does not yield a probability distribution as stated in Eq. 2. While there is no requirement to have a distribution here - a simple loss term is sufficient - the scale of this term differs compared to calibrated log-likelihoods coming from a single adversary. Hence, lambda in Eq. 3 may need to be chosen differently depending on the adversarial model. Without tuning lambda for each method, the empirical experiments seem unfair. This may also explain why, for example, the baseline method with one adversary effectively fails for Opp-L. A better comparison would be to plot the performance of the predictor of S against the performance of Y for varying lambdas. The area under this curve allows much better to compare the various methods.
There are little theoretical contributions. Basically, instead of a single adversarial model - e.g., a single-layer NN or a multi-layer NN - the authors propose to train multiple adversarial models on different views of the data. An alternative interpretation is to use an ensemble learner where each learner is trained on a different (overlapping) feature set. Though, there is no theoretical justification why ensemble learning is expected to better trade-off model capacity and robustness against an adversary. Tuning the architecture of the single multi-layer NN adversary might be as good?
In short, in the current experiments, the trade-off of the predictive performance and the effectiveness of obtaining anonymized representations effectively differs between the compared methods. This renders the comparison unfair. Given that there is also no theoretical argument why an ensemble approach is expected to perform better, I recommend to reject the paper.