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Bias and Variance in Machine Learning – Understanding the Fundamentals of ML

Imagine a world where your devices anticipate your needs before you even voice them, where complex problems are unravelled with the precision of a master puzzle solver, and where innovation doesn’t just exist but thrives autonomously. Welcome to the era of machine learning, a technological marvel that powers the extraordinary in our everyday lives. It’s the heartbeat of smart assistants, the backbone of recommendation systems, and the wizardry behind personalised experiences in the intricate world of machine learning, where the dance between two formidable foes, bias and variance, dictates the destiny of models. 


It’s a captivating tango between simplicity and complexity, where every step either underlines accuracy or veers towards unpredictability. Bias, the cautious guardian of simplicity, strives to keep models neat and tidy, while its counterpart, variance, yearns for the thrill of complexity. But wait, there’s a twist! Understanding this captivating duet isn’t just about algorithms; it’s about unfolding the secrets of crafting intelligent models that not only understand data intricacies but also pirouettes effortlessly between accuracy and adaptability. So, grab your curiosity and find all your answers related to bias and variance in machine learning here.


Table of Contents:



Errors in Machine Learning


In machine learning, errors serve as crucial metrics that gauge the accuracy and efficiency of models. An error, simply put, signifies any deviation or inaccuracy in the actions performed by the model. Essentially, it’s a reflection of how well a model predicts both the data it has been trained on and novel, unseen data. Errors play a pivotal role in determining the most suitable machine-learning model for a specific dataset. 


Types of Errors


  • Irreducible Errors

    Irreducible errors are the persistent inaccuracies within a machine learning model that stem from unknown variables. These errors persist regardless of the model’s refinement or optimisation efforts. They exist due to intrinsic limitations in the data or unpredictability in the underlying factors influencing the predictions.


  • Reducible Errors

    Contrary to irreducible errors, reducible errors can be mitigated or minimised through model adjustments and optimisation techniques. They primarily manifest in two distinct forms: bias and variance.


What is Bias in Machine Learning?


Bias is a fundamental concept that refers to the discrepancy between predicted values generated by a model and the actual or expected values. This deviation signifies an inherent error known as bias error, which stems from the model’s inability to capture the true relationship within the dataset.


During the training phase, machine learning models scrutinise data to discern underlying patterns, subsequently using these patterns to make predictions. Bias creeps in as these models learn from the dataset. It originates from the assumptions embedded within the model, simplifying the target function for easier learning. 


Types of Bias


  • Low Bias

    A model with low bias operates with minimal assumptions regarding the form of the target function. This allows for greater flexibility and adaptability, enabling the model to capture intricate relationships within the data. Models with low bias tend to be more complex, accommodating a wider array of features and patterns.

  • High Bias

    Conversely, high bias arises when a model makes substantial assumptions about the target function, simplifying its structure. This often leads to an oversimplified representation, causing the model to overlook crucial features within the dataset. Consequently, a high-bias model struggles to perform adequately on new, unseen data due to its inability to encompass the complexity present in the dataset.


Bias in Algorithms


  • High Bias Algorithms

    Linear algorithms such as Linear Regression, Linear Discriminant Analysis, and Logistic Regression typically exhibit high bias. Their simplicity enables rapid learning but restricts their capacity to encapsulate intricate relationships within the data.

  • Low Bias Algorithms

    In contrast, algorithms like Decision Trees, k-Nearest Neighbors, and Support Vector Machines often possess low bias. Their complexity allows for a more nuanced understanding of the dataset, enabling them to capture a broader range of features and patterns.


Ways to Reduce High Bias in Machine Learning


High bias in machine learning models can hinder their ability to capture the intricate relationships within a dataset, leading to oversimplified predictions. Overcoming high bias often involves employing strategies that enhance the model’s complexity and capacity to learn from data effectively.


  • Deepening Model Complexity: Consider increasing the complexity of the model, especially in cases where the current model structure is too simplistic. For instance, in neural networks, adding more hidden layers or neurons can help in capturing nuanced patterns within the data. Complex models like Polynomial Regression for non-linear datasets, Convolutional Neural Networks (CNN) for image processing, or Recurrent Neural Networks (RNN) for sequence learning can be employed to address high bias effectively.


  • Feature Expansion: Augmenting the number of features used to train the model can enhance its ability to comprehend the underlying patterns in the data. By incorporating more relevant features, the model gains a broader perspective, potentially reducing bias by capturing more intricate relationships.


  • Adjust Regularization Techniques: Regularisation methods such as L1 or L2 regularisation are crucial for preventing overfitting and improving generalisation. However, in cases of high bias, the strength of regularisation can be excessive, limiting the model’s ability to learn complex patterns. Reducing the strength of regularisation or even removing it entirely might help improve the model’s performance by allowing it to capture more nuanced relationships present in the data.


  • Enhance Training Dataset: A larger training dataset provides the model with more diverse examples to learn from, potentially reducing bias. Increasing the size of the training data can expose the model to a wider array of scenarios and patterns, aiding it in better generalisation and understanding of the underlying relationships.


Employing these strategies can assist in mitigating high bias in machine learning models, allowing them to better comprehend complex relationships within datasets and improve their predictive performance. However, it’s essential to strike a balance between model complexity and generalisation, avoiding overfitting while ensuring the model captures essential patterns within the data. Experimentation and fine-tuning based on the specific characteristics of the dataset are key to effectively reducing bias and enhancing model accuracy.


What is Variance in Machine Learning?


In machine learning, variance plays a pivotal role in assessing the stability and generalizability of a model’s predictions when exposed to different training datasets. It delineates the extent of variation that the model’s predictions would exhibit if trained on diverse sets of data. Essentially, variance measures how much a random variable deviates from its expected value.


Types of Variance:


  • Low Variance

    Low variance indicates minimal fluctuations or differences in the predictions of the target function when the training data changes. A model with low variance tends to maintain consistency in its predictions across various training datasets. Models exhibiting low variance usually generalise well to unseen data, showcasing a stable and reliable performance. They demonstrate a robust understanding of the underlying relationships between inputs and output variables.

  • High Variance

    On the other hand, high variance denotes substantial variations in the predictions of the target function when trained on different datasets. Models with high variance showcase a wide divergence in predictions, indicating a sensitivity to variations in the training data.

    High variance models tend to overfit the training data, learning excessively and intricately from it. While performing admirably on the training dataset, these models falter when faced with new, unseen data, resulting in higher error rates during testing.


Variance in Algorithms


  • Low Variance Algorithms

    Algorithms such as Linear Regression, Logistic Regression, and Linear Discriminant Analysis typically exhibit low variance. They offer stable predictions across different datasets and demonstrate good generalisation capabilities.

  • High Variance Algorithms

    On the other hand, algorithms like Decision Trees, Support Vector Machines, and K-nearest neighbours tend to have high variance. These algorithms, often non-linear in nature, possess greater flexibility in fitting the model, leading to higher variance and a propensity for overfitting.


Ways to Reduce the Variance in Machine Learning


Variance, a crucial aspect of model performance, can be effectively managed through a variety of techniques aimed at reducing the fluctuations in predictions and enhancing the model’s generalisation capabilities.


  • Understanding Model Fit: Employ cross-validation techniques to assess model performance across various subsets of the data. This method aids in identifying instances of overfitting or underfitting, guiding the fine-tuning of hyperparameters to strike a balance between bias and variance.


  • Refining Model Complexity: Opt for feature selection to isolate and include only the most relevant features. By reducing unnecessary complexities, this approach streamlines the model, mitigating variance and enhancing its ability to capture essential patterns within the data.


  • Controlling Model Complexity: Utilize regularisation methods like L1 or L2 regularisation to constrain model complexity. These techniques penalise large coefficients, thereby curbing overfitting and reducing variance, ultimately improving the model’s generalisation ability.


  • Harnessing Collective Strength: Embrace ensemble methods like bagging, boosting, and stacking, which amalgamate multiple models. These methods leverage diverse models to enhance generalisation performance, thereby diminishing variance by integrating diverse perspectives and reducing overfitting tendencies.


  • Trimming Complexity: Simplify the model architecture by reducing the number of parameters or layers, especially in complex models like neural networks. This streamlining process aids in curbing overfitting tendencies, ultimately decreasing variance and enhancing generalisation.


  • Preventing Overfitting: Implement early stopping techniques to prevent overfitting in deep learning models. By halting the training process when the model’s performance on the validation set plateaus, this method prevents excessive learning, thereby reducing variance and improving generalisation.


Employing a combination of these strategies can effectively mitigate variance in machine learning models. Striking a balance between model complexity and generalisation ensures optimal model performance, enhancing the model’s predictive capabilities on both training and unseen data. Experimentation and fine-tuning based on the specific characteristics of the dataset are key to effectively reducing variance while maintaining model accuracy.


What is the Difference Between Bias and Variance?

Bias and variance in machine learning have been differentiated in the table below:


Bias Variance
Represents errors due to oversimplified assumptions about the data Signifies errors due to the model’s sensitivity to fluctuations in training data
Arises from a model’s inability to capture the true underlying relationships Stems from excessive complexity and overfitting to training data
Results in underfitting, lacking complexity to comprehend data nuances This leads to overfitting, capturing noise and irrelevant patterns
Low bias indicates a simplistic model, making substantial assumptions High variance reflects a model’s ability to learn intricacies but struggle with generalization
Common in linear algorithms such as Linear Regression, Logistic Regression Associated with non-linear algorithms like Decision Trees, Support Vector Machines
Addressed by increasing model complexity or adding more features Mitigated through regularisation, simplifying the model, or ensemble methods
Enhancing the model’s capacity to capture intricate relationships Focusing on stability and generalisation to unseen data
Can cause models to miss important patterns in the data Can make models overly sensitive to fluctuations, leading to poor generalisation


Bias and variance are two essential aspects in evaluating the performance of machine learning models. While bias represents errors due to simplistic assumptions, variance signifies errors due to the model’s sensitivity to fluctuations in the training data. Balancing these two factors is crucial to creating models that generalise well while capturing essential patterns within the data.


Different Combinations of Bias-Variance

Understanding the interplay between bias and variance is pivotal in evaluating a machine learning model’s performance. Various combinations of bias and variance define the characteristics and predictive capabilities of these models. Here are different combinations of Bias-Variance:


Low-Bias, Low-Variance:

  • Ideal Scenario: This represents an optimal model where both bias and variance are minimal.
  • Characteristics: Accurate and consistent predictions across different datasets.
  • Practical Feasibility: However, achieving this perfect balance is challenging in real-world scenarios.


Low-Bias, High-Variance:

  • Nature of Predictions: While predictions might be accurate on average, they lack consistency.
  • Reason Behind: This arises when the model becomes too complex, learning intricate details from the data, leading to overfitting.
  • Impact: Despite potentially accurate predictions, the model struggles with generalisation to unseen data.


High-Bias, Low-Variance:

  • Nature of Predictions: Consistent but inaccurately predicting on average.
  • Cause: This scenario occurs when the model is too simplistic or lacks the capacity to capture the underlying complexities of the data.
  • Implication: The model might overlook crucial patterns, leading to underfitting problems and performing poorly on both training and test data.


High-Bias, High-Variance:


  • Prediction Characteristics: Inconsistent and inaccurate predictions on average.
  • Consequences: This scenario combines the limitations of both high bias and high variance, resulting in poor model performance.
  • Outcome: The model fails to capture essential patterns and is overly sensitive to fluctuations, leading to both inaccuracies and inconsistencies.


Understanding these combinations is crucial in model evaluation and selection. While striving for low bias and low variance is ideal, achieving this balance is often a trade-off. Models must strike the right equilibrium, minimising both bias and variance to achieve accurate and consistent predictions while generalising well to new data. Balancing these factors ensures robust and reliable machine-learning models that perform optimally across diverse datasets.


The Bias-Variance Trade-off

When constructing a machine learning model, it’s crucial to manage both bias and variance to prevent instances of overfitting or underfitting. A model that’s overly simplistic, with fewer parameters, tends to exhibit low variance and high bias. Conversely, a model with a higher parameter count often showcases high variance and low bias. Thus achieving a harmonious equilibrium between these errors, bias and variance. It is termed the Bias-Variance trade-off.


For optimal predictive accuracy, machine learning algorithms ideally require a reduction in both variance and bias. However, this proves challenging due to the inherent relationship between bias and variance:


  • Decreasing variance tends to increase bias
  • Reducing bias often leads to an increase in variance


This trade-off remains a pivotal consideration in supervised learning. The goal is to develop a model that accurately captures the underlying regularities within the training data while effectively generalising to unseen datasets. Striking this balance concurrently is unfeasible. A high variance model might excel with the training data but could overfit, especially when dealing with noisy data. Conversely, a biased model generates a simpler representation, potentially missing crucial data patterns. Thus, the challenge lies in identifying an optimal middle ground between bias and variance to craft an effective model.


Ultimately, the bias-variance trade-off revolves around pinpointing this sweet spot, a balanced intersection between bias and variance errors, to develop an optimal model.


Long Story Short


Understanding the pivotal interplay between bias and variance is paramount in the field of machine learning. By comprehending the delicate balance between these factors, individuals can craft robust models that excel in predictive accuracy and generalisation. The Accelerator Program in Artificial Intelligence and Machine Learning at Hero Vired equips aspiring enthusiasts with the necessary tools and expertise to navigate these complexities effectively. On this transformative journey with Hero Vired, you’ll delve deep into the realms of AI and ML, honing your skills to create innovative solutions and shape the future of technology. Join to unravel the mysteries of bias and variance while mastering the art of crafting intelligent and impactful machine learning models.





Bias introduces consistent errors within the ML model, embodying a simpler model that might not align with specific needs. Conversely, variance introduces errors that manifest as discrepancies in predictions, potentially identifying trends or data points that lack existence.
Overfitting denotes an unfavourable tendency in machine learning, arising when a model accurately predicts outcomes within its training data but falters when applied to new data. Data scientists initiate the predictive process by training machine learning models on established datasets.
A model showcasing low variance and significant bias tends to underfit the target, whereas a model with high variance and minimal bias tends to overfit the target. A high-variance model might accurately depict the dataset but risks overfitting, especially when encountering noisy or unrepresentative training data.
Bias arises within a machine learning model when an algorithm is applied but fails to fit accurately. Variance represents the degree of alteration in the estimation of the target function when employing different training data. It delineates the disparity between predicted values and their actual counterparts.
In the depicted illustration, it's evident that elevated bias correlates with heightened errors in both the testing and training sets. Conversely, when encountering high variance, the model demonstrates strong performance on the testing set, showcasing low error rates yet exhibiting elevated errors within the training set.

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