How to Label Data for Machine Learning

Ben Cook • Posted 2023-03-31

Machine learning has revolutionized the world of technology, playing a crucial role in various applications, from self-driving cars and facial recognition systems to language translation and sentiment analysis. The success of machine learning models largely depends on the quality and quantity of data they are trained on. In particular, labeled data, which consists of input-output pairs, is essential for the effective training of supervised learning models. These models learn to identify patterns in the input data that map to specific outputs, enabling them to make accurate predictions for new, unseen data. This blog post aims to provide a comprehensive guide on how to label data for machine learning, focusing on both computer vision and natural language processing applications.

Computer vision deals with the understanding and interpretation of visual data, such as images or videos. A computer vision model is designed to perform tasks such as image classification, object detection, and semantic segmentation. The data these models rely on consist of images or videos annotated with information, such as the presence of specific objects, their locations (typically with a bounding box or key point), or the relationships between different elements in a scene. The images and videos are the raw data, whereas the annotations show the model what it should be predict for new examples.

Natural language processing (NLP), on the other hand, focuses on the analysis and understanding of human language in the form of text or speech. Machine learning models in NLP are trained to perform tasks such as sentiment analysis, named entity recognition, and text summarization. Like other forms of artificial intelligence, NLP models rely on labeled data, which consists of text or speech annotated with relevant information, such as sentiment labels, entities, or summaries. High-quality labeled data is critical for training NLP models that can effectively understand and process human language.

The machine learning training process generally involves feeding a model with labeled data so it can learn to recognize patterns and make predictions based on those patterns. To ensure the model achieves reasonable performance, it is crucial to have a large dataset containing diverse and representative samples of the problem domain. Building a large dataset often requires careful planning, multiple data collection strategies, and data augmentation techniques that can artificially expand and enhance the original data. Ensuring dataset diversity and quality helps to minimize bias and improve the model’s generalization capabilities.

Several commercial services and open source options are available to help with the data labeling process, catering to different annotation types, budgets, and turnaround times. Choosing the right service for your project can save time and resources while ensuring the highest quality of labeled data. Additionally, it’s often a good idea to hire an external team of data labelers that specialize in annotating unstructured data for machine learning algorithms.

In this blog post, we will discuss the machine learning training process, the importance of building a large dataset, and the various techniques used for labeling data in computer vision and natural language processing applications. We will also provide an overview of commercial data labeling services and their features, as well as best practices for ensuring the highest quality labels, which maximizes the chance of success of your machine learning project.

The Machine Learning Training Process

Understanding Supervised Learning

Supervised learning is the dominant approach in practical, applied machine learning, where models are trained on labeled data to make predictions or perform specific tasks for your use case. In supervised learning, the input data, also known as features or independent variables, are paired with the correct output, called labels or dependent variables. The goal is for the model to learn the relationship between the input and output so it can make accurate predictions on new, unseen data.

Training and Validation Sets

The dataset used for supervised learning is typically divided into two parts: the training set and the validation (or test) set. The training set is used to train the model, while the validation set helps evaluate the model’s performance on unseen data. This division allows for assessing the model’s generalization capabilities and helps prevent overfitting, a scenario where the model “memorizes” the training dataset, performing well during training but poorly on new, unseen data.

Model Evaluation Metrics

To measure the performance of a machine learning model, various evaluation metrics are used depending on the task. For classification tasks, common metrics include accuracy, precision, recall, and F1 score. In regression tasks, mean absolute error, mean squared error, and R-squared are often employed. These tasks can be used on image data or on text data. For NLP tasks specifically, metrics such as BLEU, ROUGE, and METEOR can be used to assess the quality of generated text. Regardless of the metric used, the goal is to allow for quantitative evaluation of model performance, guiding the selection of the best model and hyperparameters.

Hyperparameter Tuning and Model Selection

Hyperparameters are configuration settings of the model that are not learned during training but are set before the training process. They can significantly impact the model’s performance, and finding the optimal combination of hyperparameters is a crucial step in the machine learning development process. Techniques such as grid search, random search, and Bayesian optimization are commonly used for hyperparameter tuning although in practice, good old fashioned trial and error can work surprisingly well. During this process, various models are trained using different hyperparameter combinations, and their performance is evaluated using the validation set. The model with the best performance is then selected for deployment or further fine-tuning.

Iterative Process and Continuous Improvement

Finally, it’s important to note that machine learning model development does not take place in a vacuum. And while accurate results are important, they are probably not the only thing you care about. In practice, if you want to impact real world business processes, you also need to make sure the ML models you build benefit end users. This means the process can involve multiple rounds of training, evaluation, refinement based on feedback from the validation set or domain experts and experimentation inside your product. Data scientists are likely to be a key part of the model training and development process, but it usually takes a multi-disciplinary team to make a large real-world impact with machine learning.

Furthermore, as new data becomes available or as the problem domain evolves, it is essential to retrain and update the model to maintain its accuracy and relevance for end users. By continuously refining and expanding the labeled data, the model’s performance can be improved over time, ensuring that it remains effective in addressing the problem it was designed to solve.

Importance of Building a Large, High-Quality Dataset

Data as the Key to Success

The quality and quantity of labeled data play a critical role in determining the performance of neural networks and other machine learning models. In general, the more labeled data available for training, the better the model’s performance. As long as the quality of the labels is sufficiently high, a large, diverse dataset will help the model learn the underlying patterns and relationships in the data, enabling it to make accurate predictions on new, unseen data.

Data Augmentation Techniques

Data augmentation is a technique used to artificially expand the size of a dataset by creating new instances through various transformations. For computer vision tasks, common data augmentation techniques include rotation, scaling, flipping, and color manipulation. In NLP, techniques like synonym replacement, random insertion, random deletion, and back translation can be employed. Data augmentation can help improve the model’s performance by providing more training examples, reducing overfitting, and increasing the model’s robustness to variations in the input data.

Active Learning and Incremental Improvement

Active learning is an approach that involves iteratively selecting the most informative samples from a pool of unlabeled data for labeling and adding them to the training set. This strategy can help optimize the data labeling process by focusing on the most valuable examples. As new, informative samples are added to the training set, the model can be retrained and fine-tuned, leading to continuous improvement in its performance.

Balancing Quality and Cost

Although having a large, high-quality dataset is crucial for building effective machine learning models, it is essential to balance the quality of the labeled data with the cost and effort required to create it. Utilizing efficient labeling tools, automation, and active learning strategies can help reduce the cost and time involved in building a labeled dataset, making the machine learning development process more manageable and cost-effective.

Labeling Techniques for Computer Vision and Natural Language Processing

Computer Vision Labeling Techniques

Image Classification

In image classification tasks, annotators assign one or multiple labels to an entire image. This process involves creating a predefined set of categories and having annotators select the most appropriate one(s) for each image.

Object Detection

Object detection requires annotators to draw bounding boxes around specific objects within an image and assign a label to each box. This technique is commonly used for tasks that involve detecting and identifying multiple objects within an image.

Semantic Segmentation

Semantic segmentation involves dividing an image into segments and assigning a label to each segment. Annotators use pixel-level annotations to identify and differentiate between different objects or regions in an image. This technique is often used for tasks that require a more detailed understanding of the scene, such as autonomous vehicle navigation or medical image analysis.

Instance Segmentation

Instance segmentation combines object detection and semantic segmentation by identifying individual object instances within an image while also assigning a label to each pixel belonging to the object. This technique is useful for tasks that need to differentiate between multiple instances of the same object class.

Natural Language Processing Labeling Techniques

Text Classification

Text classification is a common NLP task where annotators assign one or multiple labels to a given text. Examples include sentiment analysis, topic classification, and intent detection.

Named Entity Recognition (NER)

NER involves identifying and labeling specific entities within a text, such as names of people, organizations, locations, dates, and numerical values. Annotators highlight the relevant text spans and assign a predefined entity label to each.

Part-of-Speech (POS) Tagging

POS tagging requires annotators to label individual words or tokens within a text according to their grammatical function, such as nouns, verbs, adjectives, and adverbs. This technique is commonly used for tasks like syntactic parsing and machine translation.

Relation Extraction

Relation extraction involves identifying and labeling relationships between entities in a text. Annotators analyze the context and determine the type of relationship between the entities, such as “person-organization” or “location-event.”

Coreference Resolution

Coreference resolution aims to identify when different words or phrases within a text refer to the same entity. Annotators must recognize and link pronouns, possessive forms, and other referring expressions to their respective antecedents.

Data Labeling Best Practices

To ensure the quality and consistency of labeled data for both computer vision and natural language processing tasks, it is essential to establish clear guidelines and provide annotators with detailed instructions. Regularly reviewing and updating these guidelines, as well as implementing quality control measures, such as inter-annotator agreement checks and active learning, can help improve the overall quality of the labeled data.

Commercial Data Labeling Services and Tools

Overview of Commercial Services

Several commercial services and tools are available to help create labeled datasets for machine learning applications. These platforms offer various features, including data management, annotation tools, and automation capabilities. Some also provide access to a workforce of annotators to label the data, which can save time and effort for businesses and researchers.

Amazon SageMaker Ground Truth

Amazon SageMaker Ground Truth is a service that helps users build high-quality labeled datasets for machine learning tasks. The platform offers built-in annotation tools for various tasks, including object detection, image segmentation, and text classification. It also allows users to incorporate their own custom annotation tools. Ground Truth can integrate with Amazon Mechanical Turk, providing access to a large pool of annotators, or users can use their own workforce for labeling tasks.


Labelbox is a popular data labeling platform that provides tools for creating and managing labeled datasets. It supports a wide range of annotation tasks, including image, video, and text labeling. Labelbox offers a user-friendly interface, customizable workflows, and quality assurance features. Users can either label the data themselves or collaborate with their team, or they can use Labelbox’s managed workforce to handle the labeling process.


Appen is a company that specializes in providing data annotation services for machine learning applications. They offer a platform called Appen Connect, which allows users to manage their data labeling projects, access a global workforce of annotators, and monitor the quality of the labeled data. Appen’s services cover various domains, including computer vision, natural language processing, and audio transcription.

Open Source Tools

In addition to commercial services, several open-source data labeling tools are available. My personal favorite is the Computer Vision Annotation Tool (CVAT). These tools provide basic annotation capabilities and can be a cost-effective alternative for smaller projects or those with limited budgets. However, they may lack some features and support compared to commercial platforms and importantly, you will need to host the tool yourself.

Best Practices for Data Labeling

Establish Clear Guidelines

Defining Label Categories

Clearly define the categories or labels that annotators should use when labeling data. Provide specific examples and explanations of each label, ensuring that they are mutually exclusive and exhaustive, covering all possible cases that may be encountered.

Annotation Instructions

Provide comprehensive instructions that outline the labeling process, including step-by-step guidance and best practices for annotation. Include visual examples, case studies, and tips to help annotators understand the context and expectations.

Quality Control Measures

Inter-Annotator Agreement

Assess the consistency of annotations by comparing the work of different human labelers on the same dataset. Calculate inter-annotator agreement metrics, such as Cohen’s Kappa or Fleiss’ Kappa, to measure the level of agreement and identify areas for improvement.

Regular Reviews and Feedback

Conduct regular reviews of the labeled data to ensure annotators are following guidelines and maintaining high-quality annotations. Provide feedback and address any questions or concerns that annotators may have. This iterative process helps improve the quality of annotations over time.

Active Learning

Leverage active learning techniques to prioritize the most informative and challenging examples for annotators to label. This approach helps to improve model performance by focusing on samples that are most likely to improve the model’s understanding of the problem.

Training and Support for Annotators

Comprehensive Onboarding

Provide annotators with a thorough onboarding process, including training on labeling tools, guidelines, and best practices. Ensure that they understand the objectives and expectations of the project.

Ongoing Support and Communication

Maintain open lines of communication with annotators, addressing any questions or concerns they may have during the labeling process. Encourage them to ask questions and provide feedback on the guidelines and instructions.

Data Labeling Tools and Infrastructure

Selecting the Right Tools

Choose data labeling tools and platforms that are well-suited for your specific tasks and requirements. Consider factors such as ease of use, scalability, and the ability to collaborate and manage annotators.

Data Security and Privacy

Ensure that the tools and infrastructure you use for data labeling adhere to data security and privacy best practices. Protect sensitive information by implementing access controls, encryption, and data anonymization techniques where necessary.

Continuous Improvement

Iterative Process

Treat data labeling as an iterative process, continuously refining guidelines, instructions, and quality control measures based on feedback from annotators and model performance metrics.

Adapt to New Challenges

As your project evolves and new challenges emerge, adapt your data labeling strategy and guidelines accordingly. Stay up-to-date with the latest research and best practices in data labeling to ensure the ongoing success of your machine learning project.


In this blog post, we have covered the essential aspects of data labeling for machine learning, including its importance in both computer vision and natural language processing tasks. High-quality labeled data plays a critical role in the success of machine learning models, as it helps train algorithms to generalize effectively and perform well on real-world tasks.

We discussed the training process, emphasizing the need for a large, diverse, and representative dataset to achieve optimal model performance. As machine learning models, particularly deep learning algorithms, require vast amounts of data to learn complex patterns, the quality and quantity of labeled data directly impact the model’s ability to make accurate predictions.

Furthermore, we explored various commercial data labeling services that offer specialized expertise and resources to handle large-scale annotation projects. These services provide a valuable option for businesses and researchers that require high-quality labeled data but may not have the necessary resources, time, or expertise in-house.

In our discussion of labeling techniques, we highlighted the differences between computer vision and natural language processing tasks, emphasizing the unique challenges that each domain presents. We delved into various techniques used in each domain, such as bounding boxes, semantic segmentation, and part-of-speech tagging, to provide a better understanding of the diverse approaches required for different types of data.

We also shared best practices for data labeling, which are essential for ensuring high-quality annotations and minimizing errors. Establishing clear guidelines, implementing quality control measures, providing training and support for annotators, selecting the right tools, and continuously improving the labeling process are all crucial aspects of a successful data labeling project.

In conclusion, data labeling is a foundational step in the development of machine learning models, as it enables algorithms to learn from examples and make sense of the vast amounts of data they process. By following best practices and leveraging the expertise of commercial data labeling services when necessary, businesses and researchers can build robust, high-performing models that drive innovation and improve decision-making across a wide range of applications.

As machine learning continues to advance and become more integrated into our daily lives, the demand for high-quality labeled data will only grow. It is essential for organizations and individuals involved in machine learning projects to recognize the importance of data labeling and invest time and resources in this critical aspect of model development.

By understanding the nuances of data labeling in both computer vision and natural language processing tasks, as well as the various techniques and best practices available, you can build a strong foundation for your machine learning project. In doing so, you will set yourself up for success and increase the likelihood of achieving your desired outcomes, whether that involves developing a cutting-edge computer vision system, a sophisticated natural language understanding model, or any other innovative application of machine learning.

With the knowledge and insights gained from this blog post, you are now well-equipped to embark on your data labeling journey, ensuring the highest quality annotations and paving the way for successful machine learning projects that deliver significant value and impact.