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Title: Deep Anomaly Detection with Outlier Exposure

Paper content:
It is important to detect anomalous inputs when deploying machine learning systems.
The use of larger and more complex inputs in deep learning magnifies the difficulty of distinguishing between anomalous and in-distribution examples.
At the same time, diverse image and text data are available in enormous quantities.
We propose leveraging these data to improve deep anomaly detection by training anomaly detectors against an auxiliary dataset of outliers, an approach we call Outlier Exposure (OE).
This enables anomaly detectors to generalize and detect unseen anomalies.
In extensive experiments on natural language processing and small- and large-scale vision tasks, we find that Outlier Exposure significantly improves detection performance.
We also observe that cutting-edge generative models trained on CIFAR-10 may assign higher likelihoods to SVHN images than to CIFAR-10 images; we use OE to mitigate this issue.
We also analyze the flexibility and robustness of Outlier Exposure, and identify characteristics of the auxiliary dataset that improve performance.
Machine Learning systems in deployment often encounter data that is unlike the model's training data.
This can occur in discovering novel astronomical phenomena, finding unknown diseases, or detecting sensor failure.
In these situations, models that can detect anomalies (Liu et al., 2018; Emmott et al., 2013) are capable of correctly flagging unusual examples for human intervention, or carefully proceeding with a more conservative fallback policy.Behind many machine learning systems are deep learning models (Krizhevsky et al., 2012) which can provide high performance in a variety of applications, so long as the data seen at test time is similar to the training data.
However, when there is a distribution mismatch, deep neural network classifiers tend to give high confidence predictions on anomalous test examples (Nguyen et al., 2015) .
This can invalidate the use of prediction probabilities as calibrated confidence estimates (Guo et al., 2017) , and makes detecting anomalous examples doubly important.Several previous works seek to address these problems by giving deep neural network classifiers a means of assigning anomaly scores to inputs.
These scores can then be used for detecting outof-distribution (OOD) examples (Hendrycks & Gimpel, 2017; Lee et al., 2018; Liu et al., 2018) .
These approaches have been demonstrated to work surprisingly well for complex input spaces, such as images, text, and speech.
Moreover, they do not require modeling the full data distribution, but instead can use heuristics for detecting unmodeled phenomena.
Several of these methods detect unmodeled phenomena by using representations from only in-distribution data.In this paper, we investigate a complementary method where we train models to detect unmodeled data by learning cues for whether an input is unmodeled.
While it is difficult to model the full data distribution, we can learn effective heuristics for detecting out-of-distribution inputs by exposing the model to OOD examples, thus learning a more conservative concept of the inliers and enabling the detection of novel forms of anomalies.
We propose leveraging diverse, realistic datasets for this purpose, with a method we call Outlier Exposure (OE).
OE provides a simple and effective way to consistently improve existing methods for OOD detection.Through numerous experiments, we extensively evaluate the broad applicability of Outlier Exposure.
For multiclass neural networks, we provide thorough results on Computer Vision and Natural Language Processing tasks which show that Outlier Exposure can help anomaly detectors generalize to and perform well on unseen distributions of outliers, even on large-scale images.
We also demonstrate that Outlier Exposure provides gains over several existing approaches to out-of-distribution detection.
Our results also show the flexibility of Outlier Exposure, as we can train various models with different sources of outlier distributions.
Additionally, we establish that Outlier Exposure can make density estimates of OOD samples significantly more useful for OOD detection.
Finally, we demonstrate that Outlier Exposure improves the calibration of neural network classifiers in the realistic setting where a fraction of the data is OOD.
Our code is made publicly available at https://github.com/hendrycks/outlier-exposure.
Extensions to Multilabel Classifiers and the Reject Option.
Outlier Exposure can work in more classification regimes than just those considered above.
For example, a multilabel classifier trained on CIFAR-10 obtains an 88.8% mean AUROC when using the maximum prediction probability as the OOD score.
By training with OE to decrease the classifier's output probabilities on OOD samples, the mean AUROC increases to 97.1%.
This is slightly less than the AUROC for a multiclass model tuned with OE.
An alternative OOD detection formulation is to give classifiers a "reject class" (Bartlett & Wegkamp, 2008) .
Outlier Exposure is also flexible enough to improve performance in this setting, but we find that even with OE, classifiers with the reject option or multilabel outputs are not as competitive as OOD detectors with multiclass outputs.
In addition to size and realism, we found diversity of D OE out to be an important factor.
Concretely, a CIFAR-100 classifier with CIFAR-10 as D OE out hardly improves over the baseline.
A CIFAR-10 classifier exposed to ten CIFAR-100 outlier classes corresponds to an average AUPR of 78.5%.
Exposed to 30 such classes, the classifier's average AUPR becomes 85.1%.
Next, 50 classes corresponds to 85.3%, and from thereon additional CIFAR-100 classes barely improve performance.
This suggests that dataset diversity is important, not just size.
In fact, experiments in this paper often used around 1% of the images in the 80 Million Tiny Images dataset since we only briefly fine-tuned the models.
We also found that using only 50,000 examples from this dataset led to a negligible degradation in detection performance.
Additionally, D OE Improves Calibration.
When using classifiers for prediction, it is important that confidence estimates given for the predictions do not misrepresent empirical performance.
A calibrated classifier gives confidence probabilities that match the empirical frequency of correctness.
That is, if a calibrated model predicts an event with 30% probability, then 30% of the time the event transpires.
In this paper, we proposed Outlier Exposure, a simple technique that enhances many current OOD detectors across various settings.
It uses out-of-distribution samples to teach a network heuristics to detect new, unmodeled, out-of-distribution examples.
We showed that this method is broadly applicable in vision and natural language settings, even for large-scale image tasks.
OE can improve model calibration and several previous anomaly detection techniques.
Further, OE can teach density estimation models to assign more plausible densities to out-of-distribution samples.
Finally, Outlier Exposure is computationally inexpensive, and it can be applied with low overhead to existing systems.
In summary, Outlier Exposure is an effective and complementary approach for enhancing out-of-distribution detection systems.

assistant: text: OE teaches anomaly detectors to learn heuristics for detecting unseen anomalies; experiments are in classification, density estimation, and calibration in NLP and vision settings; we do not tune on test distribution samples, unlike previous work