1 Introduction
Anomaly detection (a.k.a. outlier detection)
(Hodge and Austin, 2004; Chandola et al., 2009; Aggarwal, 2015) aims to discover rare instances that do not conform to the patterns of majority. It has been amply studied in recent works (Liu et al., 2017; Li et al., 2017; Perozzi et al., 2014; Zong et al., 2018; Zhou and Paffenroth, 2017; Maurus and Plant, 2017; Zheng et al., 2017; Siffer et al., 2017), with solutions inspired by extreme value theory (Siffer et al., 2017), robust statistics (Zhou and Paffenroth, 2017) and graph theory (Perozzi et al., 2014).Unsupervised anomaly detection is a subarea of outlier detection which aims to discover these rare instances in an already ‘contaminated’ dataset. It is a specially hard task, where there is usually no information on what these rare instances are and most works use heuristics/approximations to discover these anomalies, providing an anomaly score
for each instance in this dataset.In this work, we first show that unsupervised anomaly detection is an undecidable problem, requiring priors to be assumed on the anomaly distribution; we then argue in favor of a new approach to anomaly detection, called active anomaly detection (Section 2). We propose a new learning layer, called here Universal Anomaly Inference (UAI), that can be applied on top of any unsupervised anomaly detection system based on deep learning to transform it in an active anomaly detection system (Section 3). We also present experiments showing the performance of our active systems vs unsupervised/semisupervised ones under similar budgets in both synthetic and real datasets (Section 4). Finally, we visualize our models learned latent representations, comparing them to unsupervised models’ ones and analyze our model’s performance for different numbers of labels (Appendix C).
2 Problem Definition
Grubbs (1969) defines an outlying observation, or outlier, as one that appears to deviate markedly from other members of the sample in which it occurs. Hawkins (1980) states that an outlier is an observation that deviates so much from other observations as to arouse suspicion that it was generated by a different mechanism. While Chandola et al. (2009) says that normal data instances occur in high probability regions of a stochastic model, while anomalies occur in the low probability regions of the stochastic model.
Following these definitions, specially the one from (Hawkins, 1980)
, we assume there is a probability density function from which our ‘normal’ data instances are generated:
(1) 
where is an instance’s available information^{1}^{1}1, in our notation, is the information known about a data instance. This can be further composed of what would actually be and in a supervised setting, such as an image and its corresponding class label. We will reference this as and here. and is a label saying if the point is anomalous or not. There is also a different probability density function from which anomalous data instances are sampled:
(2) 
In this problem, a dataset would be composed of both normal and anomalous instance, being sampled from a probability distribution that follows:
(3) 
where is an usually small constant representing the probability of a random data point being anomalous (), this constant can be either known a priori or not.
Chandola et al. (2009) divides anomaly detection learning systems in three different types:

Supervised: You are given curated training/test sets where labels of normal/anomalous instances are known. This case is similar to an unbalanced supervised classification setting:

SemiSupervised: You are given a curated training set which only contains normal instances and need to identify anomalous instances in a test set. This problem can also be called novelty detection:

Unsupervised: You are given a dataset which contains both normal and anomalous instances and must find the anomalous instances in it. There is no concept of a test set since anomalous instances must be sorted in the dataset itself:
2.1 Unsupervised Anomaly Detection
In this work we will focus on unsupervised anomaly detection. In this problem, then, there is a dataset composed of both normal and anomalous instances. Having this set of datapoints we want to find a subset which is composed of the anomalous instances.
The probability distribution is a mixture of distributions and Dasgupta et al. (2005) states that, for a mixture of distributions that overlap very closely, it may be impossible to learn the individual distributions beyond a certain accuracy threshold. In the sequence we show that it is impossible to recover from for any small without a prior on the anomalies probability distribution.
Lemma 1.
Mixture probability lemma. Consider two independent arbitrary probability distributions and . Given only a third distribution composed of the weighted average of the two:
and considering
as the residual probability distribution hyperplanes:
Without further assumptions on (without a prior on its probability distribution), we only know that and .^{2}^{2}2The proofs for all lemmas and theorems presented here can be found in Appendix D
Lemma 2.
Extreme mixtures lemma. Consider two independent arbitrary probability distributions and . Given only a third probability distribution composed of the weighted mixture of the two, and for a small , we can find a small residual hyperplane , which tends to .
(4) 
We can also find a very large residual hyperplane for , which tends to:
(5) 
Theorem 3.
No free anomaly theorem. Consider two independent arbitrary probability distributions and . For a small number of anomalies , gives us no further knowledge on the distribution of :
From Theorem 3 we can conclude that, without a prior on the anomalies distribution, unsupervised anomaly detection is an undecidable problem. A more tangible example of this can be seen in Figure 1, where we present a synthetic data distribution composed of three classes of data clustered in four visibly separable clusters. Anomaly detection is a mainly undecidable problem in this setting without further information, since its impossible to decide if the low density cluster is composed of anomalies or the anomalies are the unclustered low density points (or a combination of both).
If we used a high capacity model to model the data distribution in Figure 1, the low density points (Right) would probably be detected as anomalous. If we used a low capacity model, the cluster (Center) would probably present a higher anomaly score. In real settings, network invasion attacks (anomalies) are usually clustered data points, while health insurance frauds can be either clustered or scattered (low density) points. In clinical data, some low density clusters may indicate diseases (anomalies), while other low density clusters may be caused by uncontrolled factors in the data, such as high performance athletes, for example. We want to be able to distinguish between anomalies and ‘uninteresting’ low probability points.
3 Model
The usual strategy for solving unsupervised anomaly detection problems is training a parameterized model to capture the full data distribution
(e.g. a PCA, or AutoEncoder), and, since
is, by definition, a small constant, assuming and assuming points with low probability are anomalous (Zhou and Paffenroth, 2017). An anomaly score is then defined as . There are three main problems with this strategy:
if anomalous items are more common than desired, might be a poor approximation of ;

if anomalous items are tightly clustered in some way, high capacity models may learn to identify that cluster as a high probability region;

since we only have access to , Theorem 3 states that its impossible to recover the separate distributions and without further information/assumptions on their probability distributions.
Most unsupervised anomaly detection systems also rely on further verification of the results by human experts, due to their uncertain performance. Being mostly used as a ranking system to get high probability instances in the top of a ‘list’ to be further audited by these experts.
From Theorem 3, we conclude it is impossible to have an universal and reliable unsupervised anomaly detection system, while we know that most such systems already rely on the data being later audited by human experts. These arguments together argue in favor of an active learning strategy for anomaly detection, including the auditor experts in the system’s training loop. Thus, anticipating feedback and benefiting from it to find further anomalous instances, which results in a more robust system.
Having an extremely unbalanced dataset in this problem () is also another justification for an active learning setting, which has the potential of requiring exponentially less labeled data than supervised settings (Settles, 2012).
3.1 Active Anomaly Detection
With these motivations, we argue in favor of the new category of anomaly detection algorithms called active anomaly detection. In unsupervised anomaly detection we start with a dataset and want to rank elements in this dataset so that we have the highest possible recall/precision for a certain budget , which is the number of elements selected to be audited by an expert, with no prior information on anomaly labels.
In active anomaly detection, we also start with a completely unlabeled anomaly detection dataset , but instead of ranking anomalies and sending them all to be audited at once by our expert, we select them in small parts, waiting for the experts feedback before continuing. We iteratively select the most probable elements to be audited, wait for the expert to select their label, and continue training our system using this information, as shown in Algorithm 1. This requires the same budget as an unsupervised anomaly detection system, while having the potential of achieving a much better performance.
With this in mind, we develop the Universal Anomaly Inference (UAI) layer. This layer can be incorporated on top of any deep learning based white box anomaly detection system which provides an anomaly score for ranking anomalies. It takes as input both a latent representation layer (), created by the model, and its output anomaly score (
), and passes it through a classifier to find an item’s anomaly probability.
(6) 
This is motivated by recent works stating learned representations have a simpler statistical structure (Bengio et al., 2013), which makes the task of modeling this manifold and detecting unnatural points much simpler (Lamb et al., 2018)
. In this work, we model the UAI layer using a simple logistic regression as our classifier, but any architecture could be used here:
(7) 
where
is a linear transformation,
is a bias term andis the sigmoid function. We learn the values of
andusing backpropagation with a cross entropy loss function, while allowing the gradients to flow through
, but not through , since might be nondifferentiable. For the rest of this document, we will refer to the networks with a UAI layer as UaiNets.4 Experiments
In this section, we test our new UAI layer on top of two distinct architectures: a Denoising AutoEncoder (DAE, with
) and a Classifier (, with), which use standard multi layer perceptrons. Both architectures are described in details in Appendix
A.1. To test our algorithm we start by analyzing its performance on synthetic datasets with very different properties, presented in Section 4.1. We then present results using UaiNets on real anomaly detection datasets, shown in Section 4.2.4.1 Synthetic Data
When designing experiments, we had the objective of showing that our model can work with different definitions of anomaly, while completely unsupervised models will need, by definition, to tradeoff accuracy in one setting for accuracy in the other. With this in mind, we used the MNIST dataset and defined four sets of experiments:^{3}^{3}3Implementation details, such as the used architecture and hyperparameters can be found in Appendix A, as well as further details about the synthetic MNIST datasets. Using MNIST for the generation of synthetic anomaly detection datasets follows recent works (Zhou and Paffenroth, 2017; Zhai et al., 2016).

MNIST_{0}: For the first set of experiments, we reduced the presence of the digit class to only of its original number of samples, making it only of the dataset samples. The s still present in the dataset had its class randomly changed to and were defined as anomalies.

MNIST_{02}: The second set of experiments follows the same dataset construction, but we reduce the number of instances of numbers , and , changing the labels of the remaining items in these categories to , and again defining them as anomalous. In this dataset anomalies composed of the dataset.

MNIST_{hard}: The third set of experiments aims to test a different type of anomaly. In order to create this dataset, we first trained a weak one hidden layer MLP classifier on MNIST and selected all misclassified instances as anomalous, keeping them in the dataset with their original properties ( and ). In this dataset anomalies composed of the dataset.

MNIST_{pca}: In this set of experiments, for each image class (), we used a PCA to reduce the dimensionality of MNIST images () to and selected the instances with the largest reconstruction error as anomalies. We kept all instances in the dataset with their original properties ( and ) and in this dataset anomalies composed of the dataset.
Figure 1(a) presents results for MNIST_{0}. On this dataset, we can see that has similar results to only for the first items selected, with having already selected almost all 600 anomalies in this dataset with a budget , while plateaus after selecting around 450 anomalies and has difficulty finding the last one hundred ones.^{4}^{4}4Due to lack of space we only report full results here, but the same plots zoomed in for small budgets () can be found in Appendix B.1. Analogously, produces similar results to for a budget of up to (when is actually using to select items), but after this cold start period even outperforms , which does much better than , achieving a performance close to perfect together with .
(Color online) Results for different MNIST experiments. Lines represent median of five runs with different seeds and confidence intervals represent max and min results for each budget
.In Figure 1(b) we see similar trends on MNIST_{02}, where the model has so much difficulty to select the last items that it actually does worse than random. This further supports our claim that high capacity models can overfit to some anomalous clusters, not being able to identify them as anomalous. , on the contrary, can easily identify them and has a similar or better performance than for large budgets.
Figure 1(c) presents results for the harder task of identifying the different anomalies present in MNIST_{hard}. On this dataset we see all algorithms have more difficulty finding anomalies, with and outperforming and for budgets and respectively. We also see that, after this hot start, actually becomes the worst between the four methods, having a hard time in finding more than 600 of the approximately anomalies. At the same time, after their cold start, and fare well on this task, with having clearly the best results on this task.
Finally, the results for the task of identifying anomalies present in MNIST_{pca} are presented in Figure 1(d). On this dataset we clearly see that and fare substantially better than their underlining models and .
The main conclusion from these experiments is that, even though our algorithm might not get better results than its underlying model for every budgetdataset pair, it is robust to different types of anomalies, which is not the case for the underlying completely unsupervised models. While gives really good results in MNIST_{0} and MNIST_{02} datasets, it does not achieve the same performance in MNIST_{hard} and MNIST_{pca}, which might indicate it is better at finding clustered anomalies than low density ones. At the same time, has really good results for MNIST_{pca}, acceptable results for MNIST_{hard}, and bad ones for MNIST_{0} and MNIST_{02}, which indicates it is better at finding low density anomalies than clustered ones. Nevertheless, both UaiNets are robust in all four datasets, being able to learn even on datasets which are hard for their underlying models, although they might have a colder start to produce results.^{5}^{5}5We also report the same experiments with similar results on Appendix B.3 for the MNISTFashion dataset.
4.2 Real Data
Here we analyze our model’s performance on public benchmarks composed of real anomaly detection datasets. We employ four datasets in our analysis: KDDCUP (Lichman, 2013); Thyroid (Lichman, 2013); Arrhythmia (Lichman, 2013); and KDDCUPRev (Lichman, 2013). We use them in the same manner as described in (Zong et al., 2018) and further statistics on the datasets can be seen in Table 1. We compare our algorithm against: OCSVM (Chen et al., 2001); DAE (Vincent et al., 2008); DCN (Yang et al., 2017); DAGMM (Zong et al., 2018); and LODAAAD (Das et al., 2016).^{6}^{6}6Further descriptions of these datasets and baselines can be found in Appendix A.3, as well as descriptions of the used architectures and hyperparameters.
# Dimensions  # Instances  # Anomalies  Anomaly Ratio  

KDDCUP  120  20%  
Thyroid  6  3,772  93  2.5% 
Arrhythmia  274  452  66  15% 
KDDCUPRev  120  20% 
Table 2 presents results for these real datasets. In these experiments, OCSVM, DCN and DAGMM were trained on a semisupervised anomaly detection setting, using clean/cleaner datasets during training, DAE was trained in an unsupervised setting, while LODAAAD and were trained in an active anomaly detection setting. We can clearly see from these results that DAE produces fairly bad results for all datasets analyzed here, nevertheless, even using a simple architecture as its underlying model, produces similar results to the best baselines on the four datasets, even when the baselines were trained in completely clean training sets. also presents better results than LODAAAD, which is similarly trained in an active anomaly detection setting.
KDDCUP  Arrhythmia  
Train Set  Precision  Recall  F1  Train Set  Precision  Recall  F1  
OCSVM  0%  0.75  0.85  0.80  0%  0.54  0.41  0.46 
DCN  0%  0.77  0.78  0.78  0%  0.38  0.39  0.38 
DAGMM  0%  0.93  0.94  0.94  0%  0.49  0.51  0.50 
DAGMM  0%  0.93  0.94  0.94  0%  0.49  0.51  0.50 
DAGMM  5%  0.88  0.89  0.89  3%  0.45  0.47  0.46 
DAGMM  20%  0.42  0.43  0.43  15%  0.45  0.46  0.46 
LODAAAD  20%  0.88  0.88  0.88  15%  0.45  0.45  0.45 
DAE  20%  0.39  0.39  0.39  15%  0.35  0.35  0.35 
20%  0.94  0.94  0.94  15%  0.47  0.47  0.47  
Thyroid  KDDCUPRev  
Train Set  Precision  Recall  F1  Train Set  Precision  Recall  F1  
OCSVM  0%  0.36  0.42  0.39  0%  0.71  0.99  0.83 
DCN  0%  0.33  0.32  0.33  0%  0.29  0.29  0.29 
DAGMM  0%  0.48  0.48  0.48  0%  0.94  0.94  0.94 
DAGMM  0%  0.44  0.45  0.44  0%  0.94  0.94  0.94 
DAGMM  0.5%  0.29  0.29  0.29  5%  0.32  0.36  0.33 
DAGMM  2.5%  0.45  0.46  0.46  20%  0.31  0.31  0.31 
LODAAAD  2.5%  0.51  0.51  0.51  20%  0.83  0.83  0.83 
DAE  2.5%  0.09  0.09  0.09  20%  0.16  0.16  0.16 
2.5%  0.57  0.57  0.57  20%  0.91  0.91  0.91 
are results from our implementation of DAGMM. Unfortunately, we were not able to reproduce their results in Thyroid. For more detailed results, standard deviations and comparison to other baselines see Appendix
B.25 Related Works
Anomaly Detection
This field has been amply studied and good overviews can be found in (Hodge and Austin, 2004; Chandola et al., 2009). Although many algorithms have been recently proposed, classical methods for outlier detection, like LOF Breunig et al. (2000) and OCSVM (Schölkopf et al., 2001), are still used and produce good results. Recent work on anomaly detection has focused on statistical properties of “normal” data to identify these anomalies, such as Maurus and Plant (2017), which uses Benford’s Law to identify anomalies in social networks, and (Siffer et al., 2017), which uses Extreme Value Theory to detect anomalies. Other works focus on specific types of data, (Zheng et al., 2017) focuses on spatially contextualized data, while (Perozzi et al., 2014; Perozzi and Akoglu, 2016; Li et al., 2017; Liu et al., 2017)
focus on graph data. Recently, energy based models
(Zhai et al., 2016) and GANs (Schlegl et al., 2017) have been successfully used to detect anomalies, but autoencoders are still more popular in this field. Zhou and Paffenroth (2017) propose a method to train robust autoencoders, drawing inspiration from robust statistics (Huber, 2011) and more specifically robust PCAs, (Yang et al., 2017)focuses on clustering, and trains autoencoders that generate latent representations which are friendly for kmeans. The work most similar to ours is DAGMM
(Zong et al., 2018), where they train a deep autoencoder and use its latent representations, together with its reconstruction error, as input to a second network, which they use to predict the membership of each data instance to a mixture of gaussian models, training the whole model endtoend in an semisupervised manner for novelty detection.Active Anomaly Detection
In (Pelleg and Moore, 2005)
, the authors solve the rarecategory detection problem by proposing an active learning strategy to datasets with extremely skewed distributions of class sizes.
Abe et al. (2006) reduces outlier detection to classification using artificially generated examples that play the role of potential outliers and then applies a selective sampling mechanism based on active learning to the reduced classification problem. In (Görnitz et al., 2013), the authors proposed a SemiSupervised Anomaly Detection (SSAD) method based in Support Vector Data Description (SVDD)
(Tax and Duin, 2004), which he expanded to a semisupervised setting, where he accounts for the presence of labels for some anomalous instances, and with an active learning approach to select these instances to label. The most similar prior work to ours in this setting is (Das et al., 2016), which first describes Active Anomaly Detection (AAD) as a general approach to this problem and proposes an algorithm that can be employed with any ensemble methods based on random projections. Our work differs from these prior works mainly in that we show unsupervised anomaly detection is an undecidable problem and further formalize and motivate the proposed Active Anomaly Detection framework, contextualizing it with other anomaly detection settings. Our work also differs from them in our proposed model, which can be assembled on top of any anomaly detection Deep Learning architecture to make it work in an active anomaly detection setting.6 Discussions and Future Work
We proposed here a new architecture, Universal Anomaly Inference (UAI), which can be applied on top of any deep learning based anomaly detection architecture. We show that, even on top of very simple architectures, like a DAE, UaiNets can produce similar/better results to stateoftheart unsupervised/semisupervised anomaly detection methods.
We further want to make clear that we are not stating our method is better than any of our baselines (DAGMM, DCN, DSEBMe, or OCSVM) and our contributions are orthogonal to theirs. We are proposing a new approach to this hard problem which can be built on top of them, this being our main contribution in this work. We formalized active anomaly detection as an approach to unsupervised anomaly detection, giving both theoretical and practical arguments in favor of it, arguing that, in most practical settings, there would be no detriment to using this instead of a fully unsupervised approach.
Important future directions for this work are using the UAI layers confidence in its output to dynamically choose between either directly using its scores, or using the underlying unsupervised model’s anomaly score to choose which instances to audit next. Another future direction would be testing new architectures for UAI layers, in this work we restricted all our analysis to simple logistic regression UAI layers. A third important future work would be analyzing the robustness of UaiNets to mistakes being made by the labeling experts. Finally, making this model more interpretable, so that auditors could focus on a few “important” features when labeling anomalous instances, could increase labeling speed and make their work easier.
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Appendix A Experiments Descriptions
In this section we give detailed descriptions of the experiments. Section A.1 presents the used model architectures for both and models, as well as and . Section A.2 presents details on the synthetic MNIST datasets and on the hyperparameters used for the experiments. Finally, Section A.3 contains detailed descriptions on the used datasets, baselines and experimental settings for the experiments on real anomaly detection datasets.
a.1 Model Architectures
To show our algorithm can be assembled on top of any deep learning model, we tested it using two simple but very different anomaly detection models. The first model we test it on top of is a normal Denoising AutoEncoder (DAE).
A DAE is a neural network mainly composed by an encoder, which transforms the input into a latent space, and a decoder, which reconstructs the input using this latent representation, typically having a loss function that minimizes the reconstruction error
norm:(8) 
where both and are usually feed forward networks with the same number of layers, is a dimensional latent representation and
is a zero mean noise, sampled from a Gaussian distribution with a
standard deviation. When used in anomaly detection, the reconstruction error is usually used as an approximation of the inverse of an item’s probability, and as its anomaly score:(9) 
We then create a network by assembling the proposed UAI layer on top of the DAE:
(10) 
where is the classifier chosen for the UAI layer. Another typical approach to unsupervised anomaly detection is, when given a dataset with labeled data , training a classifier () to predict from ^{8}^{8}8Note that, even though in this problem we have class labels (), we have no anomaly labels of objects (), so this is still an unsupervised anomaly detection problem. and using the crossentropy of an item as an approximation to the inverse of its probability distribution:
(11) 
where
is typically a feed forward neural network with
layers, from which we can use its last hidden layer () as the data’s latent representation to be used in the .(12) 
For all experiments in this work, unless otherwise stated, the DAE’s encoder and decoder had independent weights and we used both the and models with hidden layers and hidden sizes . This means the latent representations provided to the UAI layers are . We implemented all experiments using TensorFlow [Abadi et al., 2016], and used a learning rate of
, batch size of 256 and the RMSprop optimizer with the default hyperparameters. For the active learning models, we pretrain the DAE/Class model for
optimization steps, select items to be labeled at a time, and further train for iterations after each labeling call. To deal with the cold start problem, for the first 10 calls of select_top, we use the base anomaly score () of the / model to make this selection, using the UAI one for all later labeling decisions.a.2 Synthetic Data
Detailed statistics on the synthetic MNIST datasets can be seen in Table 3. MNIST_{0} and MNIST_{02} were mainly generated with the purpose of simulating the situation in Figure 1 (Center), where anomalies were present in sparse clusters. At the same time, MNIST_{hard} and MNIST_{pca} were designed to present similar characteristics to the situation in Figure 1 (Right), where anomalous instances are in sparse regions of the data space.
# Dimensions  # Classes  # Instances  # Anomalies  Anomaly Ratio  

MNIST_{0}  784  9  1.1%  
MNIST_{02}  784  7  4.2%  
MNIST_{hard}  784  10  3.5%  
MNIST_{pca}  784  10  5% 
a.3 Real Data
For these experiments, we used the same datasets as [Zong et al., 2018] and preprocessed them in the same manner as them:

KDDCUP [Lichman, 2013]: The KDDCUP99 10 percent dataset from the UCI repository. Since it contains only 20% of instances labeled as “normal” and the rest as “attacks”, “normal” instances are used as anomalies, since they are in a minority group. This dataset contains 34 continuous features and 7 categorical ones. We transform these 7 categorical features into their one hot representations, and obtain a dataset with 120 features.

Thyroid [Lichman, 2013]
: A dataset containing data from patients which can be divided in three classes: normal (not hypothyroid), hyperfunction and subnormal functioning. In this dataset, we treat the hyperfunction class as an anomaly, with the other two being treated as normal. It can be obtained from the ODDS repository.
^{9}^{9}9http://odds.cs.stonybrook.edu 
Arrhythmia [Lichman, 2013]: This dataset was designed to create classification algorithms to distinguish between the presence and absence of cardiac arrhythmia. In it, we use the smallest classes (3, 4, 5, 7, 8, 9, 14, and 15) as anomalies and the others are treated as normal. This dataset can also be obtained from the ODDS repository.

KDDCUPRev [Lichman, 2013]: Since “normal” instances are a minority in the KDDCUP dataset, we keep all “normal” instances and randomly draw “attack” instances so that they compose 20% of the dataset.
We compare our algorithm against:

OCSVM [Chen et al., 2001]
: Oneclass support vector machines are a popular kernel based anomaly detection method. In this work, we employ it with a Radial Basis Function (RBF) kernel.

DAE [Vincent et al., 2008]: Denoising Autoencoders are autoencoder architectures which are trained to reconstruct instances from noisy inputs.

DCN [Yang et al., 2017]: Deep Clustering Network is a stateoftheart clustering algorithm. Its architecture is designed to learn a latent representation using deep autoencoders which is easily separable when using kmeans.

DAGMM [Zong et al., 2018]: Deep Autoencoding Gaussian Mixture Model is a stateoftheart model for semisupervised anomaly detection which simultaneously learns a latent representation, using deep autoencoders, and uses both this latent representation and the autoencoder’s reconstruction error to learn a Gaussian Mixture Model for the data distribution.
Since there is no validation/test set in unsupervised anomaly detection, we cannot tune hyperparameters on a validation set. Because of this, to make the DAE baselines more competitive, we got the results for several different hyperparameter configurations and present only the best among them. This indeed is an unfair approach, but we only do it to our baselines, while for our proposed algorithm we keep hyperparameters fixed for all experiments. We even keep our hidden sizes fixed to
on thyroid, which only contains 6 features per instance, since our objective here is not getting the best possible results, but showing the robustness of our approach. The only hyperparameter change we make in UAI networks is that, since there are fewer anomalies in Arrhythmia and Thyroid datasets, we set our active learning approach to choose 3 instances at a time, instead of 10.Results for OCSVM, DCN, and DAGMM were taken from [Zong et al., 2018], while results labeled as DAGMM* are from our implementation of this model and follow the same procedures as described in [Zong et al., 2018] and using the same architectures and hyperparameters, being trained in a semisupervised setting. The results for LODAAAD were run using the code made available by the authors and with the same steps as .^{10}^{10}10https://github.com/shubhomoydas/ad_examples
Appendix B Detailed Results
In this section, we present more detailed results for both the synthetic (Section B.1) and real (Section B.2) anomaly detection datasets, which couldn’t fit on the main paper due to lack of space. We also present results for synthetic anomaly detection experiments on FashionMNIST (Section B.3).
b.1 Detailed Results on MNIST
We present here detailed results for small budgets () on the MNIST experiments, with graphs zoomed in for these budget values. Analyzing Figure 3 we see that for some of these datasets UaiNets present a cold start, producing worse results for small budgets. Nonetheless, after this cold start, they produce better results in all MNIST experiments. An interesting future work would be to measure the confidence in the UaiNet’s prediction to dynamically choose between using its anomaly score or the underlying network’s one, which could solve/reduce this cold start problem.
b.2 Detailed Results on Real Data
Table 4 presents detailed results for experiments on real datasets, showing standard deviations for the experiments we ran. In this table we also compare our results to:

DSEBMe [Zhai et al., 2016]: Deep Structured Energy Based Models are anomaly detection systems based on energy based models [LeCun et al., 2006], which are a powerful tool for density estimation. We compare here against DSEBMe, which uses a data instance’s energy as the criterion to detect anomalies.

DSEBMr [Zhai et al., 2016]: Deep Structured Energy Based Model with the same architecture and training procedures as DSEBMe, but using an instance’s reconstruction error as the criterion for anomaly detection.
Dataset  Method  Anomalies in  Precision  Recall  F1 

Train Set  
KDDCUP  OCSVM  0%  0.7457  0.8523  0.7954 
OCSVM  5%  0.1155  0.3369  0.1720  
PAE  0%  0.7276  0.7397  0.7336  
DSEBMr  0%  0.1972  0.2001  0.1987  
DSEBMe  0%  0.7369  0.7477  0.7423  
DSEBMe  5%  0.5345  0.5375  0.5360  
DCN  0%  0.7696  0.7829  0.7762  
DCN  5%  0.6763  0.6893  0.6827  
DAGMM  0%  0.9297  0.9442  0.9369  
DAGMM  5%  0.8504  0.8643  0.8573  
DAGMM  0%  0.9290 (0.0344)  0.9435 (0.0349)  0.9362 (0.0346)  
DAGMM  5%  0.8827 (0.0682)  0.8965 (0.0693)  0.8896 (0.0688)  
DAGMM  20%  0.4238 (0.0187)  0.4304 (0.0190)  0.4271 (0.0188)  
LODAAAD  20%  0.8756 (0.1255)  0.8756 (0.1255)  0.8756 (0.1255)  
DAE  20%  0.3905 (0.2581)  0.3905 (0.2581)  0.3905 (0.2581)  
20%  0.9401 (0.0191)  0.9401 (0.0191)  0.9401 (0.0191)  
Thyroid  OCSVM  0%  0.3639  0.4239  0.3887 
PAE  0%  0.1894  0.2062  0.1971  
DSEBMr  0%  0.0404  0.0403  0.0403  
DSEBMe  0%  0.1319  0.1319  0.1319  
DCN  0%  0.3319  0.3196  0.3251  
DAGMM  0%  0.4766  0.4834  0.4782  
DAGMM  0%  0.4375 (0.1926)  0.4468 (0.1967)  0.4421 (0.1947)  
DAGMM  0.5%  0.2875 (0.1505)  0.2936 (0.1537)  0.2905 (0.1521)  
DAGMM  2.5%  0.4542 (0.2995)  0.4638 (0.3059)  0.4590 (0.3027)  
LODAAAD  2.5%  0.5097 (0.0712)  0.5097 (0.0712)  0.5097 (0.0712)  
DAE  2.5%  0.0860 (0.0725)  0.0860 (0.0725)  0.0860 (0.0725)  
2.5%  0.5742 (0.0582)  0.5742 (0.0582)  0.5742 (0.0582)  
Arrhythmia  OCSVM  0%  0.5397  0.4082  0.4581 
PAE  0%  0.4393  0.4437  0.4403  
DSEBMr  0%  0.1515  0.1513  0.1510  
DSEBMe  0%  0.4667  0.4565  0.4601  
DCN  0%  0.3758  0.3907  0.3815  
GADMM  0%  0.4909  0.5078  0.4983  
GADMM  0%  0.4902 (0.0514)  0.5051 (0.0530)  0.4975 (0.0522)  
GADMM  3%  0.4530 (0.0573)  0.4666 (0.0591)  0.4597 (0.0582)  
GADMM  15%  0.4500 (0.0597)  0.4636 (0.0615)  0.4567 (0.0606)  
LODAAAD  15%  0.4485 (0.0136)  0.4485 (0.0136)  0.4485 (0.0136)  
DAE  15%  0.3485 (0.0392)  0.3485 (0.0392)  0.3485 (0.0392)  
15%  0.4727 (0.0225)  0.4727 (0.0225)  0.4727 (0.0225)  
KDDCUPRev  OCSVM  0%  0.7148  0.9940  0.8316 
PAE  0%  0.7835  0.7817  0.7826  
DSEBMr  0%  0.2036  0.2036  0.2036  
DSEBMe  0%  0.2212  0.2213  0.2213  
DCN  0%  0.2875  0.2895  0.2885  
GADMM  0%  0.9370  0.9390  0.9380  
GADMM  0%  0.9391 (0.1553)  0.9391 (0.1553)  0.9391 (0.1553)  
GADMM  5%  0.3184 (0.1358)  0.3559 (0.2096)  0.3341 (0.1658)  
GADMM  20%  0.3051 (0.1059)  0.3053 (0.1060)  0.3052 (0.1059)  
LODAAAD  20%  0.8339 (0.1081)  0.8339 (0.1081)  0.8339 (0.1081)  
DAE  20%  0.1626 (0.0609)  0.1626 (0.0609)  0.1626 (0.0609)  
20%  0.9117 (0.0160)  0.9125 (0.0170)  0.9121 (0.0165) 
The results presented here are averages of five runs, with standard deviations in parenthesis. In this table, results for OCSVM, PAE, DSEBMr, DSEBMe, DCN and DAGMM were taken from [Zong et al., 2018], while DAGMM
are results from our implementation of DAGMM. Unfortunately, we were not able to reproduce their results in the Thyroid dataset, getting a high variance in the results. LODAAAD does not scale well to large datasets, so to run it on KDDCUP and KDDCUPRev we needed to limit its memory about the anomalies it had already learned, forgetting the oldest ones. This reduced its runtime complexity from
to in our tests, where is the budget limit for the anomaly detection task.On this table we can see that produces better results than LODAAAD on all analyzed datasets. Our proposed method also, besides presenting results comparable to stateoftheart DAGMM trained on a clean dataset, is much more stable, having a lower standard deviation than the baselines in almost all datasets.
b.3 Experiments on FashionMNIST
In this Section, we present results for experiments on synthetic anomaly detection datasets based on FashionMNIST [Xiao et al., 2017]. To create these datasets we follow the same procedures as done for MNIST in Section 4.1, generating four datasets: FashionMNIST_{0}; FashionMNIST_{02}; FashionMNIST_{hard}; FashionMNIST_{pca}. Detailed statistics of these datasets can be seen in Table 5.
# Dimensions  # Classes  # Instances  # Anomalies  Anomaly Ratio  

FashionMNIST_{0}  784  9  1.1%  
FashionMNIST_{02}  784  7  4.0%  
FashionMNIST_{hard}  784  10  16.1%  
FashionMNIST_{pca}  784  10  5.0% 
We run experiments on these datasets following the exact same procedures as in Section 4.1. Figure 4 shows the results for FashionMNIST_{0} and FashionMNIST_{02}, while Figure 5 show the results for FashionMNIST_{hard} and FashionMNIST_{pca}. These figures show similar trends to the ones for MNIST, although algorithms find anomalies in these datasets harder to identify. Specially for FashionMNIST_{hard}, takes a long time to start producing better results than . Nevertheless, UaiNets are still much more robust than the underlying networks to different types of anomalies, producing good results in all four datasets, even when its underlying network gives weak results on that dataset.
Appendix C Further Analysis
In this section we further study UaiNets, analyzing the evolution of hidden representations and anomaly scores through training (Section
C.1), and the dependence of results on the number of audited anomalies (Section C.2).c.1 Learned Representations and Anomaly Scores
In this section, we show visualizations of the learned representations () and anomaly scores () of UaiNets’ underlying networks, presenting their evolution as more labels are fed into the network through the active learning process. With this purpose, we retrain UaiNets on both MNIST_{02} and MNIST_{hard}, with a hidden size of , so that its latent representation is one dimensional (), and plot these representations vs the anomaly scores () of the base network (either or ) for different budgets ().
Figure 6 shows the evolution of ’s underlying and . In it, we can see that initially (Figures 6 (a, d)) anomalies and normal data instances are not separable in this space. Nevertheless, with only a few labeled instances () the space becomes much easier to separate, while for the space is almost perfectly linearly separable.^{11}^{11}11Gifs showing these models evolution can be found in https://homepages.dcc.ufmg.br/~tpimentel/paper_imgs/uai/hidden_vs_loss/.
Figure 7 shows the same evolution for ’s underlying and . In it, we can also see the same patterns, as initially anomalies and normal data instances are not separable, but with a few labeled instances anomalies become much more identifiable.
The main conclusion taken from these visualizations is how the gradient flow through is important, since it helps the network better separate data in these spaces, allowing good performance even when the underlying networks are not good at identifying a specific type of anomaly.
c.2 Anomaly Choices Evolution through Training
This experiments aim at showing how the networks choice quality evolves with the access to more labels. Here, we present the choices network would make having access to a fixed number of expert labels. With this in mind, we train the networks in the same way as in Section 4.2, but stop after reaching a specific budget (), showing the choices made up to that point, and after that with no further training.
Figure 8 shows the evolution of anomaly choices as it is fed more expert knowledge. We can see that with only a few labels it already fairs a lot better than its underlying network. In KDDCUP with only labeled instances, which is less than of the dataset, it can correctly find anomalies with a high precision, while the with no expert knowledge does a lot worse. On Thyroid and KDDCUPRev, with of the dataset labeled ( and , respectively) it finds all or almost all anomalies in the dataset correctly. The Arrhythmia dataset is a lot smaller and with few anomalies, so improves on in a smaller scale here, but it still does fairly better than the underlying network.^{12}^{12}12Gifs showing this choice evolution can be found in https://homepages.dcc.ufmg.br/~tpimentel/paper_imgs/uai/budget_evolution/
Appendix D Proofs
d.1 Lemma 1. Mixture probability lemma
Lemma 1.
Consider two independent arbitrary probability distributions and . Given only a third distribution composed of the weighted average of the two:
and considering as the residual probability distribution hyperplanes:
Without further assumptions on (without a prior on its probability distribution), we only know that and .
Proof.
Given we know that:
with and . Assuming the distribution of is independent of , and with no further assumptions on it, is random and uniform on the set of all possible probability distributions, so its probability distribution is:
where is the hyperspace containing all probability distributions, with an hypervolume . Now we can try to find :
The equality in (1) comes from the definition of the space , which is the space of all possible values of that could result in , so if , then . Equality (2) is a simple variable substitution where . (3) comes from the assumption that and are independent. Equality (4) results from and having a volume . Finally, Equality (5) is a result from the fact that .
With a similar strategy we can find :
where Equality (1) and (2) result from the fact that , given a specific value of . This completes this proof. ∎
d.2 Lemma 2. Extreme mixtures lemma
Lemma 2.
Consider two independent arbitrary probability distributions and . Given only a third probability distribution composed of the weighted mixture of the two, and for a small , we can find a small residual hyperplane , which tends to .
(13) 
We can also find a very large residual hyperplane for , which tends to:
(14) 
where is the support of a probability distribution.
Proof.
In this proof, we start with the arbitrary residual hyperplanes and find restrictions in the limits of and . For a :
For a we start with the other definition of :
This finishes this proof. ∎
d.3 Theorem 3. No free anomaly theorem
Theorem 3.
Consider two independent arbitrary probability distributions and . For a small number of anomalies , the knowledge that
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