Top 5 Outlier Detection Methods Every Data Enthusiast Must Know

Top 5 Outlier Detection Methods Every Data Enthusiast Must Know

Outliers are data points that deviate significantly from the normal distribution or projected trends within a dataset in the context of data analysis. These data points can introduce noise, modify statistical measurements, and degrade analytical model correctness. As a result, identifying and dealing with outliers is crucial for generating trustworthy insights and making data-driven decisions. Outliers can take numerous forms, including extreme values, anomalies, and data-gathering errors. They can occur as a result of measurement errors, data corruption, or events that occur rarely. Outliers, regardless of their source, have the potential to distort statistical summaries, interfere with hypothesis testing, and mislead analytical models. As a result, outlier detection methods must be employed to properly identify and handle them.

Understanding Outlier Detection in Data Analysis

Outlier detection methods automate the discovery of outliers by utilizing statistical methodologies, machine learning algorithms, or domain-specific knowledge. Statistical-based approaches, distance-based methods, clustering-based methods, and supervised or unsupervised learning algorithms are a few examples of the methods used. Every method has advantages and limitations; therefore, choosing the appropriate strategy for a certain data analysis job is crucial.

In the following sections, we will look at some outlier detection techniques that should be familiar to any data enthusiast. These techniques provide various approaches for locating and managing outliers while maintaining the validity and dependability of data analyses. Understanding and applying these techniques can help you improve your data analysis skills and make more accurate decisions.

Choosing the Right Outlier Detection Method for Your Data Analysis Project 

Outlier detection techniques differ in their advantages, constraints, and assumptions. Therefore, it is important to choose the right one based on the unique properties of your data, the goals of your analysis, and the requirements of your project. In this section, we will look at some of the most effective outlier removal methods and discuss their advantages.

1. Z-Score

The Z-score method is a statistically based approach for outlier detection. It computes the standard score, or Z-score, for each data point. It computes how many standard deviations a data point deviates from the mean of the dataset. We then set a threshold for our Z-score, and data points with Z-scores greater than it are considered outliers. An important assumption made by the Z-score method is that your data is normally distributed, making it especially useful for datasets with symmetrical patterns around the mean.

from sklearn.datasets import load_breast_cancer
from scipy import stats

threshold = 2.5
df = load_breast_cancer(as_frame=True).data
z_scores = stats.zscore(df)
outliers = df[abs(z_scores) > threshold]

The Z-score method for outlier detection has the following advantages:

  1. Ease of implementation
  2. Assumes that the data is distributed normally, which is a widely applicable assumption for situations in the real world.
  3. Offers a numerical assessment of the extremeness of each outlier based on standard deviations.

Cons of employing the Z-score method for outlier detection include the following:

  1. If your data is not normally distributed, Z-score will not be effective for detecting outliers
  2. It may be influenced by the presence of other outliers in the dataset.
  3. Depending on the dataset and context, the threshold value selection has to be done carefully

By leveraging the Z-score method as an outlier detector, you can quickly identify data points that deviate significantly from the expected statistical patterns. However, it is important to be aware of the assumptions and limitations of this method and consider alternative approaches when dealing with datasets that do not conform to the normal distribution.

2. Local Outlier Factor (LOF)

The Local Outlier Factor (LOF) algorithm calculates a data point’s local density deviation in relation to its neighbors. LOF assigns an anomaly score to each data point, indicating how likely it is to be an outlier. Outliers are points that have a high anomaly score.

LOF calculates the LOF score for each data point by comparing the local density of each data point to the local densities of its neighbors. An outlier is a data point whose local density is significantly lower than that of its neighbors. Because it considers the concept of local density, LOF is useful for datasets with a range of densities or clusters. When we use the implementation in scikit-learn, we can convert LOF scores to predictions by using the predict or fit_predict method, which assigns a value of 1 to points that are not outliers and -1 to points likely to be outliers.

from sklearn.datasets import load_breast_cancer
from sklearn.neighbors import LocalOutlierFactor
data = load_breast_cancer(as_frame=True).data
lof = LocalOutlierFactor(n_neighbours=20,contamination=0.1)
outliers = lof.fit_predict(data)
data["LOF"] = outliers

The LOF method for outlier detection has the following advantages:

  1. Effective in identifying outliers in datasets with varying densities or clusters.
  2. Doesn’t require assumptions about the underlying distribution of the data.
  3. Provides anomaly scores that can be used to rank the outliers.

Cons of employing the LOF method for outlier detection include the following:

  1. Sensitivity to the choice of parameters such as the number of neighbors (n_neighbors) and the contamination rate (contamination).
  2. Can be computationally expensive for large datasets.
  3. May require careful interpretation and adjustment of the anomaly scores threshold for outlier detection.

Data enthusiasts can identify outliers based on local density deviations and capture anomalies that display different patterns from their neighbors by using the Local Outlier Factor (LOF) method for outlier detection. To get precise outlier detection, however, parameter tuning and careful result interpretation are necessary.

3. Isolation Forest

The Isolation Forest algorithm is an effective and efficient unsupervised outlier detection tool. It operates by isolating outliers as abnormalities in a random forest structure. Unlike typical decision trees, which divide data into non-overlapping sections, the Isolation Forest method randomly selects features and splits data points until outliers are isolated into individual leaves.

 Isolation Forest assigns an anomaly score to each data point, with lower scores indicating a higher risk of being an outlier. The approach takes advantage of the fact that outliers are expected to have shorter average path lengths in the random forest, making them easier to isolate. When we use the implementation in scikit-learn, using the predict or fit_predict method will assign a value of 1 to data points unlikely to be outliers and -1 to points that are likely outliers.

from sklearn.datasets import load_breast_cancer
from sklearn.ensemble import IsolationForest
data = load_breast_cancer(as_frame=True).data
iso = IsolationForest(contamination=0.1)
outliers = iso.fit_predict(data)
data["ISO"] = outliers

The Isolation Forest method for outlier detection has the following advantages:

  • Effective in identifying outliers in high-dimensional datasets.
  • Can handle datasets with mixed variable types (numeric and categorical).
  • Efficient for processing large datasets due to its random partitioning strategy.

 Cons of employing the Isolation Forest method for outlier detection include the following:

  • Sensitivity to the choice of parameters, especially the contamination rate.
  • May require tuning of hyperparameters, such as the number of trees in the forest.
  • Interpretation of anomaly scores can be challenging.

 Data enthusiasts can quickly find anomalies in high-dimensional datasets by using the Isolation Forest method for outlier detection, making it a useful tool for many applications. However, careful parameter selection and result interpretation are essential for useful results.


DBSCAN is a density-based clustering technique that can also detect outliers. It gathers data points that are close to each other depending on a distance criterion. Outliers are data points that are far removed from any cluster. DBSCAN defines three types of data points:

  • Core Points: Data points within a specified neighborhood of a minimum number of other data points.
  • Border Points: Data points within the specified neighborhood of a core point but do not have enough neighboring points to be considered core themselves.
  • Noise Points (Outliers): Data points that are neither core nor border points.

The DBSCAN technique does not require a prior specification of the number of clusters, making it ideal for datasets with an unknown number of clusters. It identifies outliers based on their separation from dense data regions. The fit_predict method of the DBSCAN estimator fits the model to the data, and the labels_ attribute contains the cluster labels assigned to each data point. Outliers are identified as data points labeled as -1.

from sklearn.datasets import load_breast_cancer
from sklearn.cluster import DBSCAN
data = load_breast_cancer(as_frame=True).data
dbscan = DBSCAN()
outliers = dbscan.fit_predict(data[['mean radius']])
data["DBSCAN"] = outliers

The DBSCAN method for outlier detection has the following advantages:

  • Doesn’t require specifying the number of clusters in advance.
  • Effective in detecting outliers in datasets with irregular shapes and varying densities.
  • Robust to noise and able to handle datasets with complex structures.

 Cons of employing the DBSCAN method for outlier detection include the following:

  • Sensitivity to the choice of parameters, especially the eps and min_samples values.
  • Performance can degrade for high-dimensional datasets. Notice how we only used one column in the code sample above. This is because if we use higher dimensions, we end up marking almost every point as an outlier.
  • Difficulty in determining optimal parameter values for different datasets.

5. Coresets

Coresets use concepts from computational geometry to significantly reduce a dataset’s size while maintaining the original dataset’s statistical properties. This is done by computing coresets for subsections of the entire dataset, then taking unions of pairs of coresets and computing a new coreset until we are left with one coreset that represents our dataset. This approach results in a tree-like structure containing all coresets computed to get the final result, called a streaming tree or a coreset tree. You can learn more about Coresets and their use cases here. Coresets use a measure called importance or sensitivity to determine the impact of individual data points on candidate solutions for the given loss function. Higher importance values typically indicate that a data point is likely to be an outlier.

import numpy as np
from dataheroes import CoresetTreeServiceDTC
data = load_breast_cancer()
X = np.array(
y = np.array(
tree = CoresetTreeServiceDTC(optimized_for = "cleaning")
tree =,y=y)
result = tree.get_important_samples(20)

The Coreset method for outlier detection has the following advantages:

  • Computationally efficient for large datasets , unlike the other mentioned methods. Coresets are significantly more efficient for situations involving big data
  • Building a Coreset tree makes future computations faster and less computationally intensive.

Improving Data Quality with Outlier Detection Methods

Outliers can dramatically impact the effectiveness and accuracy of data analysis. They may skew statistical results, impair model performance, and result in erroneous inferences. Data enthusiasts may raise the standard of their data analysis and guarantee more precise and dependable findings by using outlier identification techniques. The following are some ways that outlier identification techniques improve data quality:

  • Identifying data errors: Outliers can often arise due to errors in data collection, entry, or transmission. By applying outlier removal methods, data enthusiasts can identify these errors and take necessary corrective actions.
  • Cleaning and preprocessing: Outlier detection is an essential step in data cleaning and preprocessing pipelines. By identifying outliers, data can be cleansed and preprocessed to ensure more accurate analysis.
  • Enhancing model performance: Outliers can adversely affect the performance of machine learning models. Anomalies not representative of the underlying patterns can lead to overfitting or biased model results.
  • ¬†Data validation and quality assurance: Methods for detecting outliers are an important resource for data validation and quality control procedures. They assist in assuring the data’s integrity and identifying abnormalities that can affect the analysis’s validity. The quality and dependability of the dataset may be confirmed by checking the data against predicted ranges or trends.
  • Gaining deeper insights: while sometimes dismissed as noise or mistakes, Outliers can occasionally give significant insights and reveal underlying patterns or oddities of interest. By using outlier detection tools, data enthusiasts can distinguish between legitimate abnormalities that deserve additional research and outliers that may be ascribed to mistakes or noise. This enables a more complete and nuanced comprehension of the data.

In conclusion, different outlier identification methods are critical in increasing data quality for data analysis. They aid in detecting data issues, contribute to pipeline cleaning and preprocessing, improve model performance, validate data, and give deeper insights. Data enthusiasts may ensure more accurate and dependable outcomes by adding these strategies to their data analysis workflows, leading to better decision-making and insightful insights from the data.

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