Home Blog Comprehensive Guide to Anomaly Detection in Machine Learning

Comprehensive Guide to Anomaly Detection in Machine Learning

Published: April 11, 2024
Writer at Plat.AI
Writer: Sona Poghosyan
Editor at Plat.AI
Editor: Ani Mosinyan
Reviewer at Plat.AI
Reviewer: Alek Kotolyan
Exploring Anomaly Detection with Machine Learning

Ever wonder how your email filters out spam or how your bank quickly spots a fraudulent transaction on your account? The hero behind these everyday protections is anomaly detection in machine learning – a smart, behind-the-scenes detective that is always on the lookout for anything that doesn’t quite fit the pattern. 

Imagine it as a super-sleuth combing through vast amounts of data, picking out the odd, the unusual, and the outright strange. Whether it’s keeping personal information safe, ensuring your health data is accurate, or even making sure the weather forecast is on point, anomaly detection plays a key role in making sense of the digital world. 

Dive into the fascinating world of anomaly detection with us and discover how it turns complex data into actionable insights, helping to keep our lives smooth and secure.

What Is Anomaly Detection in Machine Learning?

Anomaly detection is a machine learning technique used to find unusual patterns or data points that don’t fit with the rest of the information. It’s like a detective tool for data, searching through numbers and trends to spot anything that stands out as odd or out of place. 

This method is handy in many areas, such as spotting fraud in financial transactions, identifying unusual behavior in computer networks, and even detecting health issues in medical data. It helps us better understand data and make informed decisions.

How Does Anomaly Detection Work?

Anomaly detection uses machine learning algorithms to analyze patterns in data. These algorithms are trained on a dataset where the normal patterns are known, allowing them to recognize when something doesn’t match these patterns. 

Essentially, the system learns what typical data looks like and then flags anything that deviates from this norm as an anomaly. This process can be automated to scan vast amounts of data quickly, making it a powerful tool for detecting unusual occurrences in fields ranging from cybersecurity to healthcare analytics.

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Types of Anomalies

To effectively leverage anomaly detection techniques, you need to be familiar with anomalous data. These anomalies fall into the following three categories:

Global Outliers

Global outliers, also known as point anomalies, are data points that differ significantly from most of the data. These outliers do not fit within the normal range of values and are easily identifiable as they stand out when compared to the rest. 

For instance, in a dataset of average daily temperatures for a city, a single day where the temperature spikes to an extreme high or drops to an unusual low would be considered a global outlier.

Contextual Outliers

Contextual outliers are data points that only appear abnormal when considered within a specific context or environment. These anomalies are not inherently strange on their own but become noticeable when factors like time or location are taken into account. 

A significant drop in sales, for example, might be normal for a seasonal business during its off-season but would be an anomaly during peak periods. Identifying these outliers requires understanding the context in which the data exists.

Collective Outliers

Collective outliers consist of groups of data points that, together, deviate from the overall pattern of the data, although they might not appear unusual as individual observations. Unlike contextual outliers, which are identified based on their deviation within a specific context, collective outliers are identified by the abnormal pattern or behavior they present as a group, not tied to a specific context but rather their collective arrangement or sequence. 

An example of collective outliers could be a series of credit card transactions that individually seem normal but, when occurring one after another in a very short time frame, could indicate fraud. This unusual pattern, when seen as a group, raises alarms that wouldn’t be triggered by looking at each transaction on its own.

Overview of Machine Learning Anomaly Detection Algorithms

To spot these outliers, anomaly detection machine learning algorithms are designed to sift through vast amounts of data to uncover patterns and irregularities that may indicate potential issues or insights. These algorithms employ different strategies depending on the nature of the data and the specific objectives of the detection process.

Supervised Learning

Supervised learning in data anomaly detection involves training a machine learning model on a dataset where the normal and anomalous instances are clearly labeled. This method teaches the model the difference in characteristics of normal versus abnormal data, allowing it to accurately classify new, unseen instances as either normal or anomalous. 

It is particularly effective when precise labels are available, making it a reliable approach for scenarios where anomalies are well-defined and distinguishable based on historical data.

Unsupervised Learning

Unsupervised learning in anomaly detection does not require labeled data. Instead, it analyzes all data points to find any that significantly differ from the majority, considering these as potential anomalies. 

This approach is useful when you don’t have prior knowledge of what constitutes normal or abnormal behavior within the dataset. Algorithms such as clustering or neural networks are often used to automatically identify outliers by grouping similar data and spotting those that do not fit any group, making it adaptable to various and evolving data environments.

Semi-Supervised Learning

Semi-supervised learning bridges the gap between supervised and unsupervised learning by using both labeled and unlabeled data for training anomaly detection models. This approach leverages a small amount of labeled data to guide the learning process while also exploring the larger unlabeled dataset to uncover additional insights and anomalies. 

It’s particularly effective in situations where obtaining comprehensive labels is difficult or costly. Semi-supervised learning allows models to improve their accuracy and adaptability by learning from the broader data context. Thus, it can uncover subtle, complex anomalies that might be overlooked by purely supervised or unsupervised methods.

Use Cases of Anomaly Detection

Anomaly detection machine learning has a wide array of applications across various sectors, enhancing efficiency and security. Some of the most common fields which use anomaly detection machine learning are the following:

  • Finance: Anomaly detection algorithms can sift through vast amounts of transaction data in real-time to identify fraudulent activities, such as unauthorized credit card use, or detect unusual trading patterns that may suggest market manipulation.
  • Healthcare: By analyzing patient records, data anomaly detection can highlight inconsistencies or outliers in patient data, aiding in the early detection of misdiagnoses, erroneous entries, or emerging medical conditions that deviate from standard patterns.
  • Cybersecurity: Cybersecurity experts use anomaly detection to help identify suspicious activities within a network, such as unexpected access attempts, unusual data transfers, or patterns of traffic that could signal a breach or an ongoing attack.
  • Manufacturing: Within manufacturing processes, data anomaly detection tools monitor product quality by identifying defects or irregularities in production lines. By catching issues early in the process, they help maintain high-quality standards and reduce waste.

Using Supervised Learning For:

Supervised learning can be applied effectively in various contexts by leveraging labeled data to predict outcomes or classify data. Let’s take a closer look at the advantages that machine learning anomaly detection can offer in the retail industry and weather reporting:

Shops and Sales

  • Detects sudden changes in sales patterns, signaling potential issues like stock shortages or pricing errors.
  • Identifies unusual purchasing behavior, which could indicate fraudulent transactions or emerging consumer trends.
  • Monitors for inconsistencies in sales data across different locations or times, aiding in pinpointing operational inefficiencies.
  • Helps in forecasting demand more accurately by understanding past anomalies, optimizing stock levels, and reducing waste.
  • Alerts to unusual drops or spikes in sales, facilitating quick response to market changes or internal issues.

Weather Reports

  • Pinpoints unusual weather patterns, aiding in the early detection of extreme conditions like heatwaves, storms or wildfires.
  • Helps to improve accuracy and data quality in weather models by identifying and analyzing anomalies in historical weather data.
  • Enhances accuracy in weather predictions by learning from past deviations, improving preparedness for severe weather events.
  • Detects inconsistencies in weather data collection, ensuring reliability in weather monitoring systems.
  • Assists in climate research by highlighting unusual climate trends, contributing to a better understanding of climate change impacts.

Using Unsupervised Learning For:

Unsupervised learning excels in identifying patterns and anomalies without the need for labeled data, making it ideal for exploring unstructured data sets. Its applications in the security and manufacturing sectors show how unsupervised learning can uncover insights and anomalies, enhancing security measures and operational efficiency in diverse environments.

Security Alerts

  • Detects unusual patterns in network traffic, highlighting potential cybersecurity threats or attacks.
  • Identifies anomalies in system log files, which could indicate unauthorized access or malicious activities.
  • Recognizes abnormal user behaviors, suggesting compromised accounts or insider threats.
  • Flags unexpected changes in file or database access patterns, pointing to data breaches or exfiltration attempts.
  • Enables real-time surveillance of digital environments for emerging or zero-day threats, enhancing security response.

Factory Work

  • Monitors equipment performance, identifying deviations that could indicate a need for maintenance or repair.
  • Analyzes production processes to spot inefficiencies or anomalies, suggesting areas for improvement.
  • Detects unusual patterns in product quality, aiding in the early identification of defects or production issues.
  • Helps in optimizing manufacturing workflows by understanding patterns and variances in production data.
  • Assists in resource allocation by identifying unusual consumption patterns of materials or energy, contributing to more sustainable practices.

Using Semi-Supervised Learning For:

Semi-supervised learning combines the strengths of both labeled and unlabeled data to enhance learning models, making it highly effective for complex or partially documented datasets. Its capability to refine predictions and identify anomalies in nuanced or evolving scenarios means it is commonly used in the health and financial sectors.

Robot hand and human hand touching digital sphere network

Health Checks

  • Enhances disease detection by integrating labeled data on known conditions with broader patient data sets.
  • Identifies unusual patient symptoms or test results that could indicate rare or emerging health issues.
  • Supports early diagnosis and treatment by revealing patterns not immediately evident through traditional methods.
  • Aids in patient monitoring by detecting deviations in health data over time, suggesting changes in condition.
  • Facilitates research by uncovering potential correlations between various health indicators and outcomes, guiding preventive care strategies.

Spotting Scams

  • Improves fraud detection by combining known scam signatures with analysis of broader transaction data for unusual patterns.
  • Identifies emerging scam tactics by learning from both reported cases and large volumes of unlabeled data, staying ahead of scammers.
  • Helps detect phishing attempts by analyzing email characteristics against known fraud and broader email datasets.
  • Enhances accuracy in spotting fake online listings by leveraging examples of confirmed scams to identify suspicious new postings.
  • Aids in uncovering complex scam networks by detecting subtle connections and patterns across partially labeled data.

Machine Learning Algorithms for Anomaly Detection

These anomaly detection techniques are powered by specific algorithms that sift through data. Each anomaly detection model offers a unique approach to data processing and identifying outliers, underscoring the versatility and depth of machine learning in tackling anomaly detection challenges across various domains. These are some of the best anomaly detection algorithms available:

Local Outlier Factor (LOF)

The Local Outlier Factor (LOF) algorithm identifies anomalies by measuring the local density deviation of a given data point with respect to its neighbors. It calculates how isolated a point is in comparison to a surrounding neighborhood. Points that have a significantly lower density than their neighbors are considered outliers. 

The LOF algorithm is most effective in datasets with:

  • It’s great for data with many different aspects or features, helping to spot outliers by looking at the relationships between these features.
  • LOF can handle data that’s spread out unevenly, finding outliers in both crowded and empty areas.
  • It’s good at dealing with data that has intricate patterns where simple methods won’t work.
  • LOF can also find outliers in data involving physical space, like locations on a map, by examining how things are arranged and how close they are to each other.

The LOF algorithm helps doctors find patients whose information stands out significantly from others, which can be useful for spotting rare or unusual diseases early on. For instance, if a patient’s symptoms and test results are very different from people with similar health backgrounds, they’re marked as unusual. This means doctors might need to look more closely or think about special treatments. Using LOF makes it easier for doctors to diagnose accurately and take care of their patients better.

K-nearest Neighbors

The K-nearest Neighbors (KNN) algorithm detects anomalies by measuring the distance between a point and its closest neighbors. If a data point is far away from its K nearest neighbors, it’s flagged as an outlier. 

KNN is versatile, easy to implement, and most effective in datasets where:

  • The data naturally splits into groups or clusters, making it easy to see which points don’t fit into any group.
  • There’s a clear space or gap between these groups, so KNN can easily spot the points that are far away from the main clusters.
  • The data isn’t too complex or doesn’t have too many details because KNN can get confused if there’s too much information, making it hard to tell which points are truly different.

For instance, in detecting credit card fraud, KNN examines typical spending patterns – where and how much is spent. If a purchase drastically differs, such as a large amount far from usual locations or rapid successive buys, KNN flags it as potential fraud by contrasting it with regular spending, highlighting transactions that require further investigation.

Support Vector Machines

Support Vector Machines (SVM) help find and separate outliers from normal data by drawing a line (or hyperplane) between them. They are especially efficient when the unusual data is rare or significantly different from the rest of the data. 

SVMs are especially useful in datasets where:

  • Data is grouped in clear, separate categories. It is easier for SVM to draw a line between them.
  • There is data with lots of details (high-dimensional), and SVM can handle the complexity using special functions.
  • You need a strong method that won’t get confused by a few odd data points during learning. SVM can adjust to ignore these minor distractions.

In manufacturing, SVMs help check if each item is made right or has problems. They look at pictures or data from sensors to spot any issues, learning from examples of good and bad items. This way, they can catch things like odd shapes or textures, making sure only the best products go out to customers. This cuts down on waste and keeps customers happy.


Density-Based Spatial Clustering of Applications with Noise (DBSCAN) groups together data points based on how closely packed they are. Unlike LOF, which looks at how data points stick out compared to their neighbors, DBSCAN simply sees if points are in a busy area or not, focusing more on how they group together rather than if they’re different. 

This makes DBSCAN great for finding patterns in data where:

  • Points are spread out unevenly, which is tough for other tools that expect everything to be spaced out evenly.
  • The groups of points can be oddly shaped, which is fine for DBSCAN since it doesn’t expect everything to form perfect groups.
  • You don’t know how many groups there are to start with, as DBSCAN can figure out the natural groupings on its own.
  • There’s a lot of random noise or odd points scattered around. DBSCAN is good at spotting these and knowing they don’t belong to any group.

In marketing, DBSCAN groups customers by their buying habits, helping retailers identify similar shopper groups. This allows for targeted marketing campaigns tailored to specific customer preferences, boosting engagement and sales.


Think of autoencoders as experts in making mini versions of regular data and then trying to rebuild it to its original form. They train themselves using typical data, aiming to replicate it without errors. If they stumble upon something odd that doesn’t quite match up during reconstruction, they flag it as unusual, signaling it’s not like the rest.

Autoencoders work well for data that:

  • It is very detailed, like pictures, because autoencoders can simplify it while keeping the important bits.
  • Has complicated patterns that aren’t straightforward, as autoencoders, especially the advanced ones, are good at figuring out these complex details.
  • Needs simplifying or boiling down to its essence, where autoencoders help by picking out the most telling features or making a compact version of the original data.
  • Is big and doesn’t come with labels since autoencoders can learn all by themselves without needing things to be tagged. This is great when putting labels on everything would take too much time or money.

In cybersecurity, autoencoders help monitor network traffic. They learn what normal traffic looks like and can spot unusual events, like a hacker trying to get in. If the network traffic looks weird and doesn’t match what the autoencoder expects, it raises an alarm. This quick alert helps security teams act quickly to stop cyberattacks.

Bayesian Networks

Bayesian Networks are like maps that show how different pieces of information depend on each other, using arrows to connect them. In spotting something unusual, they work by understanding the chances of certain things happening together. If it turns out that seeing a specific combination of things is really rare, they flag it as odd or out of the ordinary.

This approach is most effective with datasets that:

  • Has parts that are heavily connected or depend on each other, which Bayesian Networks can map out well.
  • Includes different kinds of information, both clear-cut (like yes or no answers) and measurable (like heights or weights), since Bayesian Networks can handle both types together.
  • Can benefit from what experts already know about how the parts relate, improving its ability to predict things.
  • Involves situations where things aren’t certain or definite, like figuring out risks or making decisions when outcomes are unclear because it’s good at dealing with uncertainty.

Bayesian Networks can help spot unusual weather by looking at data like satellite images and temperature. For example, if they notice a mix of high heat, lots of moisture, and dropping pressure that’s different from usual, they might predict a big storm is coming. This way, forecasters can warn people earlier, even if usual weather models didn’t see it coming, helping everyone get ready and stay safe.

Sum Up

Machine learning for anomaly detection simplifies understanding large datasets spotting outliers that might indicate problems or opportunities. It’s key in protecting against cybersecurity threats, improving healthcare diagnostics, and preventing financial fraud. These algorithms boost efficiency and security across various sectors, leading to innovative solutions for new challenges.

Sona Poghosyan

WriterSona is a skilled writer, editor, and proofreader with years of experience in media and IT. Her work can be found in various tech, finance, and lifestyle publications. In her free time, she enjoys reading and writing about all things film and literature.

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