3,045 research outputs found
Anomaly Detection for imbalanced datasets with Deep Generative Models
Many important data analysis applications present with severely imbalanced
datasets with respect to the target variable. A typical example is medical
image analysis, where positive samples are scarce, while performance is
commonly estimated against the correct detection of these positive examples. We
approach this challenge by formulating the problem as anomaly detection with
generative models. We train a generative model without supervision on the
`negative' (common) datapoints and use this model to estimate the likelihood of
unseen data. A successful model allows us to detect the `positive' case as low
likelihood datapoints.
In this position paper, we present the use of state-of-the-art deep
generative models (GAN and VAE) for the estimation of a likelihood of the data.
Our results show that on the one hand both GANs and VAEs are able to separate
the `positive' and `negative' samples in the MNIST case. On the other hand, for
the NLST case, neither GANs nor VAEs were able to capture the complexity of the
data and discriminate anomalies at the level that this task requires. These
results show that even though there are a number of successes presented in the
literature for using generative models in similar applications, there remain
further challenges for broad successful implementation.Comment: 15 pages, 13 figures, accepted by Benelearn 2018 conferenc
Normalizing Flows for Human Pose Anomaly Detection
Video anomaly detection is an ill-posed problem because it relies on many
parameters such as appearance, pose, camera angle, background, and more. We
distill the problem to anomaly detection of human pose, thus reducing the risk
of nuisance parameters such as appearance affecting the result. Focusing on
pose alone also has the side benefit of reducing bias against distinct minority
groups. Our model works directly on human pose graph sequences and is
exceptionally lightweight ( parameters), capable of running on any
machine able to run the pose estimation with negligible additional resources.
We leverage the highly compact pose representation in a normalizing flows
framework, which we extend to tackle the unique characteristics of
spatio-temporal pose data and show its advantages in this use case. Our
algorithm uses normalizing flows to learn a bijective mapping between the pose
data distribution and a Gaussian distribution, using spatio-temporal graph
convolution blocks. The algorithm is quite general and can handle training data
of only normal examples, as well as a supervised dataset that consists of
labeled normal and abnormal examples. We report state-of-the-art results on two
anomaly detection benchmarks - the unsupervised ShanghaiTech dataset and the
recent supervised UBnormal dataset
Positive Difference Distribution for Image Outlier Detection using Normalizing Flows and Contrastive Data
Detecting test data deviating from training data is a central problem for
safe and robust machine learning. Likelihoods learned by a generative model,
e.g., a normalizing flow via standard log-likelihood training, perform poorly
as an outlier score. We propose to use an unlabelled auxiliary dataset and a
probabilistic outlier score for outlier detection. We use a self-supervised
feature extractor trained on the auxiliary dataset and train a normalizing flow
on the extracted features by maximizing the likelihood on in-distribution data
and minimizing the likelihood on the contrastive dataset. We show that this is
equivalent to learning the normalized positive difference between the
in-distribution and the contrastive feature density. We conduct experiments on
benchmark datasets and compare to the likelihood, the likelihood ratio and
state-of-the-art anomaly detection methods
Quasi Anomalous Knowledge: Searching for new physics with embedded knowledge
Discoveries of new phenomena often involve a dedicated search for a
hypothetical physics signature. Recently, novel deep learning techniques have
emerged for anomaly detection in the absence of a signal prior. However, by
ignoring signal priors, the sensitivity of these approaches is significantly
reduced. We present a new strategy dubbed Quasi Anomalous Knowledge (QUAK),
whereby we introduce alternative signal priors that capture some of the salient
features of new physics signatures, allowing for the recovery of sensitivity
even when the alternative signal is incorrect. This approach can be applied to
a broad range of physics models and neural network architectures. In this
paper, we apply QUAK to anomaly detection of new physics events at the CERN
Large Hadron Collider utilizing variational autoencoders with normalizing flow.Comment: 25 pages, 9 figure
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