Intervening on early readouts for mitigating spurious features and simplicity bias

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Posted by Rishabh Tiwari, Pre-doctoral Researcher, and Pradeep Shenoy, Research Scientist, Google Research

Machine learning models in the real world are often trained on limited data that may contain unintended statistical biases. For example, in the CELEBA celebrity image dataset, a disproportionate number of female celebrities have blond hair, leading to classifiers incorrectly predicting “blond” as the hair color for most female faces — here, gender is a spurious feature for predicting hair color. Such unfair biases could have significant consequences in critical applications such as medical diagnosis.

Surprisingly, recent work has also discovered an inherent tendency of deep networks to amplify such statistical biases, through the so-called simplicity bias of deep learning. This bias is the tendency of deep networks to identify weakly predictive features early in the training, and continue to anchor on these features, failing to identify more complex and potentially more accurate features.

With the above in mind, we propose simple and effective fixes to this dual challenge of spurious features and simplicity bias by applying early readouts and feature forgetting. First, in “Using Early Readouts to Mediate Featural Bias in Distillation”, we show that making predictions from early layers of a deep network (referred to as “early readouts”) can automatically signal issues with the quality of the learned representations. In particular, these predictions are more often wrong, and more confidently wrong, when the network is relying on spurious features. We use this erroneous confidence to improve outcomes in model distillation, a setting where a larger “teacher” model guides the training of a smaller “student” model. Then in “Overcoming Simplicity Bias in Deep Networks using a Feature Sieve”, we intervene directly on these indicator signals by making the network “forget” the problematic features and consequently look for better, more predictive features. This substantially improves the model’s ability to generalize to unseen domains compared to previous approaches. Our AI Principles and our Responsible AI practices guide how we research and develop these advanced applications and help us address the challenges posed by statistical biases.

Animation comparing hypothetical responses from two models trained with and without the feature sieve.

Early readouts for debiasing distillation

We first illustrate the diagnostic value of early readouts and their application in debiased distillation, i.e., making sure that the student model inherits the teacher model’s resilience to feature bias through distillation. We start with a standard distillation framework where the student is trained with a mixture of label matching (minimizing the cross-entropy loss between student outputs and the ground-truth labels) and teacher matching (minimizing the KL divergence loss between student and teacher outputs for any given input).

Suppose one trains a linear decoder, i.e., a small auxiliary neural network named as Aux, on top of an intermediate representation of the student model. We refer to the output of this linear decoder as an early readout of the network representation. Our finding is that early readouts make more errors on instances that contain spurious features, and further, the confidence on those errors is higher than the confidence associated with other errors. This suggests that confidence on errors from early readouts is a fairly strong, automated indicator of the model’s dependence on potentially spurious features.

Illustrating the usage of early readouts (i.e., output from the auxiliary layer) in debiasing distillation. Instances that are confidently mispredicted in the early readouts are upweighted in the distillation loss.

We used this signal to modulate the contribution of the teacher in the distillation loss on a per-instance basis, and found significant improvements in the trained student model as a result.

We evaluated our approach on standard benchmark datasets known to contain spurious correlations (Waterbirds, CelebA, CivilComments, MNLI). Each of these datasets contain groupings of data that share an attribute potentially correlated with the label in a spurious manner. As an example, the CelebA dataset mentioned above includes groups such as {blond male, blond female, non-blond male, non-blond female}, with models typically performing the worst on the {non-blond female} group when predicting hair color. Thus, a measure of model performance is its worst group accuracy, i.e., the lowest accuracy among all known groups present in the dataset. We improved the worst group accuracy of student models on all datasets; moreover, we also improved overall accuracy in three of the four datasets, showing that our improvement on any one group does not come at the expense of accuracy on other groups. More details are available in our paper.

Comparison of Worst Group Accuracies of different distillation techniques relative to that of the Teacher model. Our method outperforms other methods on all datasets.

Overcoming simplicity bias with a feature sieve

In a second, closely related project, we intervene directly on the information provided by early readouts, to improve feature learning and generalization. The workflow alternates between identifying problematic features and erasing identified features from the network. Our primary hypothesis is that early features are more prone to simplicity bias, and that by erasing (“sieving”) these features, we allow richer feature representations to be learned.

Training workflow with feature sieve. We alternate between identifying problematic features (using training iteration) and erasing them from the network (using forgetting iteration).

We describe the identification and erasure steps in more detail:

• Identifying simple features: We train the primary model and the readout model (AUX above) in conventional fashion via forward- and back-propagation. Note that feedback from the auxiliary layer does not back-propagate to the main network. This is to force the auxiliary layer to learn from already-available features rather than create or reinforce them in the main network.
• Applying the feature sieve: We aim to erase the identified features in the early layers of the neural network with the use of a novel forgetting loss, L_f , which is simply the cross-entropy between the readout and a uniform distribution over labels. Essentially, all information that leads to nontrivial readouts are erased from the primary network. In this step, the auxiliary network and upper layers of the main network are kept unchanged.

We can control specifically how the feature sieve is applied to a given dataset through a small number of configuration parameters. By changing the position and complexity of the auxiliary network, we control the complexity of the identified- and erased features. By modifying the mixing of learning and forgetting steps, we control the degree to which the model is challenged to learn more complex features. These choices, which are dataset-dependent, are made via hyperparameter search to maximize validation accuracy, a standard measure of generalization. Since we include “no-forgetting” (i.e., the baseline model) in the search space, we expect to find settings that are at least as good as the baseline.

Below we show features learned by the baseline model (middle row) and our model (bottom row) on two benchmark datasets — biased activity recognition (BAR) and animal categorization (NICO). Feature importance was estimated using post-hoc gradient-based importance scoring (GRAD-CAM), with the orange-red end of the spectrum indicating high importance, while green-blue indicates low importance. Shown below, our trained models focus on the primary object of interest, whereas the baseline model tends to focus on background features that are simpler and spuriously correlated with the label.

Feature importance scoring using GRAD-CAM on activity recognition (BAR) and animal categorization (NICO) generalization benchmarks. Our approach (last row) focuses on the relevant objects in the image, whereas the baseline (ERM; middle row) relies on background features that are spuriously correlated with the label.

Through this ability to learn better, generalizable features, we show substantial gains over a range of relevant baselines on real-world spurious feature benchmark datasets: BAR, CelebA Hair, NICO and ImagenetA, by margins up to 11% (see figure below). More details are available in our paper.

Our feature sieve method improves accuracy by significant margins relative to the nearest baseline for a range of feature generalization benchmark datasets.

Conclusion

We hope that our work on early readouts and their use in feature sieving for generalization will both spur the development of a new class of adversarial feature learning approaches and help improve the generalization capability and robustness of deep learning systems.

Acknowledgements

The work on applying early readouts to debiasing distillation was conducted in collaboration with our academic partners Durga Sivasubramanian, Anmol Reddy and Prof. Ganesh Ramakrishnan at IIT Bombay. We extend our sincere gratitude to Praneeth Netrapalli and Anshul Nasery for their feedback and recommendations. We are also grateful to Nishant Jain, Shreyas Havaldar, Rachit Bansal, Kartikeya Badola, Amandeep Kaur and the whole cohort of pre-doctoral researchers at Google Research India for taking part in research discussions. Special thanks to Tom Small for creating the animation used in this post.