13 minute read

What are the most challenging machine learning problems?

Emmanuel Ohiri

Emmanuel Ohiri

Machine learning (ML) is a trendy technology that enables computers to learn from data and perform tasks that would otherwise require human intelligence. ML is used in various fields, such as computer vision, natural language processing, recommender systems, self-driving cars, etc. However, implementing ML has its challenges. ML professionals often face many difficulties and obstacles in developing and deploying ML solutions that meet the expectations and requirements of their stakeholders. This article will discuss some of the most common and challenging ML problems and how to solve them.

1. Data Quality Issues

Machine Learning is fundamentally data-driven. The quality of the data used to train models directly impacts the performance and reliability of those models. Therefore, data quality issues pose a significant challenge in machine learning. The saying "garbage in, garbage out" holds particularly true in machine learning - no matter how sophisticated ML models are, they will fail to perform correctly if fed poor-quality data.

Common Data Quality Issues In Machine Learning

Data quality issues come in various forms, including:

  • Missing Data

Missing data is one of the most common data quality issues. Some fields in the dataset might be empty due to various reasons like errors in data collection, failure to record data, or simply because the information was unavailable.

  • Inconsistent Data

Inconsistencies can occur when data is collected from different sources or at different times. For example, the same type of information might be recorded in different formats or units.

  • Noisy Data

These are data with a large amount of irrelevant information or random variation. This noise can distort the underlying patterns in the data, making it harder for machine learning models to learn effectively.

  • Outliers

Outliers are data points that differ significantly from other observations. While some outliers represent valuable information, others can result from errors or anomalies in data collection.

  • Skewed or Biased Data

Data skewness or bias can happen when the data collected does not accurately represent the population. This could lead to models that perform well on the training data but poorly on real-world data.

2. Overfitting and Underfitting

Overfitting and underfitting are two common problems in machine learning. Overfitting occurs when a machine learning model learns the training data too well. It captures the training data's underlying patterns, noise, and outliers. As a result, while the model might perform exceptionally well on the training data, it performs poorly on new, unseen data. An overfitted model has learned the training data so well that it fails to generalise to new situations.

On the other hand, underfitting happens when a model fails to capture the underlying pattern of the data. The model is too simple to understand the complex structures in the data, resulting in poor performance on both the training and unseen data. An underfitted model hasn't learned enough from the training data.

What is overfitting and underfitting in Machine Learning?

Overfitting and underfitting are common phenomena in ML. Overfitting occurs when a model learns the training data too well, while underfitting happens when a model fails to capture the underlying pattern of the data. Regularisation, cross-validation, controlling model complexity, and ensemble methods can mitigate these issues.

How Overfitting and Underfitting Affect ML Models

An overfitted model may lead to overly optimistic results during training and validation, only to perform poorly when deployed in a real-world situation. This can lead to incorrect decisions based on its predictions and, ultimately, a lack of trust in the model. While an underfitted model will consistently perform poorly, providing no better predictions than random guessing. This reduces the model's usefulness and can also waste resources on training and deploying a model that provides no value.

3. Curse of Dimensionality

The term "curse of dimensionality" was coined by Richard Bellman in the 1960s to describe the problems that arise when working with high-dimensional data. As the number of features (or dimensions) in a dataset increases, the volume of the space grows exponentially, making the available data sparse. This sparsity is problematic for any method that requires statistical significance as it makes it difficult for models to learn effectively from the data and can lead to overfitting.

For instance, consider a unit square; it can be split into 100 smaller squares, each of side lengths 0.1. But a unit cube would need to be split into 1000 smaller cubes, each of side lengths 0.1, to achieve the same granularity. This exponential growth continues as more dimensions are added.

Implications of the Curse of Dimensionality

  • Increased Computational Cost

As the number of dimensions increases, so does the computational cost. Models take longer to train and require more computational resources, which can be especially problematic when dealing with real-time applications or large datasets.

  • Decreased Model Performance

High-dimensional data can lead to overfitting. With too many features and insufficient representative samples, models can start to fit the noise in the data rather than the underlying patterns, leading to poor generalisation to new, unseen data.

  • Loss of Intuition

Data can be visualised and understood relatively easily in three dimensions or less. However, our ability to visualise and intuitively understand the data diminishes as we move to higher dimensions. This loss of intuition can make it harder to interpret models, detect outliers, or notice patterns.

What is the curse of dimensionality?

The curse of dimensionality is a problem that arises when working with high-dimensional data, leading to increased computational cost, decreased model performance, and loss of intuition. This can be solved through dimensionality reduction techniques like PCA and t-SNE, feature selection methods like RFE, Lasso regression, tree-based model feature importance, and regularisation techniques.

4. Scalability

As datasets get larger and more complex, scalability becomes a significant issue. The ability of an algorithm to effectively handle an increase in data volume or complexity, continue learning and make precise predictions is what makes it scalable. However, many machine learning algorithms can become computationally intensive and challenging to scale as the dataset's size increases.

The scalability challenge in machine learning arises due to several reasons:

  • Increased Computational Cost: As the volume of data increases, the computational cost of training a machine learning model can increase exponentially. This is because most traditional machine learning algorithms simultaneously require access to all data points, which can be computationally expensive for large datasets.
  • Memory Limitations: Large datasets may not fit into the memory of a single machine, making it challenging to use standard machine learning algorithms that require all data to be loaded into memory.
  • Overfitting: With high-dimensional data, the risk of overfitting increases.

Solutions to Common Machine Learning Problems

These are ways to solve the most challenging Machine Learning problems:

1. How to Solve Data Quality Issues In Machine Learning

Despite these challenges, there are strategies to solve data quality issues in machine learning:

  • Data Cleaning

Data cleaning is detecting and correcting (or removing) corrupt or inaccurate records from the dataset. Techniques for handling missing data include deletion (removing rows with missing values) or imputation (replacing missing values with statistical estimates like mean, median, or mode). Outliers can be detected using methods like the Z-score or IQR method and can be handled by deleting or transforming them.

  • Data Transformation

Data transformation involves changing the scale or distribution of data to make it more suitable for analysis. This includes normalisation (scaling features to a standard range) and standardisation (reshaping the data to have a mean of 0 and a standard deviation of 1).

  • Data Integration

Data integration is the process of combining data from different sources into a unified view. This often involves resolving inconsistencies in data representation or scale.

  • Feature Engineering

Feature engineering involves creating new features from existing ones to better capture the underlying data patterns. Carefully crafted features can help models make more accurate predictions and are often more important than the choice of the model itself.

  • Data Augmentation

Data augmentation is a strategy for artificially increasing the size and diversity of the dataset. This can involve techniques like bootstrapping (sampling with replacement), SMOTE (Synthetic Minority Over-sampling Technique), or creating synthetic data.

2. Solving Overfitting and Underfitting In Machine Learning

Despite these challenges, there are several strategies to mitigate overfitting and underfitting:

  • Regularisation

Regularisation is a technique used to prevent overfitting by adding a penalty term to the loss function that discourages complex models. The two most common forms of regularisation are L1 and L2 regularisation.

L1 regularisation, also known as Lasso regularisation, adds a penalty equal to the absolute value of the magnitude of coefficients. This can lead to sparse solutions where some feature coefficients can become zero and, effectively, some features are entirely ignored by the model.

L2 regularisation, also known as Ridge regularisation, adds a penalty equal to the square of the magnitude of coefficients. This tends to distribute the weights of the coefficients evenly and creates less sparse solutions.

  • Cross-Validation

Cross-validation is a robust preventative measure against overfitting. The idea is to divide the dataset into 'k' subsets and train the model 'k' times, each time leaving out one of the subsets from training and using it as the test set. The average error across all 'k' trials is computed. This technique provides a more robust measure of model performance by ensuring that the model performs well on different subsets of data.

  • Model Complexity

The complexity of the model plays a crucial role in overfitting and underfitting. If the model is too complex, it may capture the noise in the data, leading to overfitting. On the other hand, if the model is too simple, it may fail to capture important patterns, leading to underfitting.

Therefore, choosing the right model complexity is critical. Techniques like pruning in decision trees, choosing the right degree polynomial in linear regression, or adjusting the architecture in neural networks can be used to control model complexity.

  • Ensemble Methods

Ensemble methods, which combine the predictions of several models, can also help solve overfitting and underfitting. Bagging and boosting are two popular ensemble methods.

Bagging (Bootstrap Aggregating) involves creating several subsets of the original dataset (with replacement), training a model on each, and combining their predictions. This method reduces variance and thus helps to prevent overfitting.

Boosting, on the other hand, trains models in sequence, with each new model being trained to correct the errors made by the previous ones. This method reduces bias and can help to prevent underfitting.

3. Solving the Curse of Dimensionality

Despite these challenges, there are several strategies to deal with the curse of dimensionality:

  • Dimensionality Reduction

Dimensionality reduction techniques aim to reduce the number of features in a dataset without losing important information. Techniques like Principal Component Analysis (PCA) and t-Distributed Stochastic Neighbour Embedding (t-SNE) transform the original high-dimensional data into a lower-dimensional space.

PCA finds the axes in the feature space along which the variance of the data is maximised, essentially finding the most "informative" directions in the feature space. t-SNE, on the other hand, is particularly good at preserving local structure and is often used for visualisation.

  • Feature Selection

Feature selection methods aim to find a subset of the original features most informative or relevant to the problem. Techniques like Recursive Feature Elimination (RFE), Lasso regression, or feature importance from tree-based models can be used to select the most relevant features and discard the rest.

  • Regularisation

Regularisation techniques can also help combat the curse of dimensionality by adding a penalty term to the loss function that discourages complex models with many parameters. L1 regularisation can even drive some model coefficients to zero, effectively performing feature selection.

4. Strategies to Overcome Scalability Issues

Despite these challenges, there are several strategies and approaches to tackle the scalability issue in machine learning:

Online Learning Algorithms:

Another approach is to use online learning algorithms. Unlike batch learning, online learning algorithms process the data one instance at a time (or in small batches). This makes them well-suited for large datasets and streaming data.

Online learning algorithms are computationally efficient as they update the model incrementally as each data point arrives, rather than retraining the model from scratch. This allows them to handle large datasets and learn in real-time.

Dimensionality Reduction:

Dimensionality reduction techniques, such as Principal Component Analysis (PCA) or t-Distributed Stochastic Neighbor Embedding (t-SNE), can also tackle the scalability issue. These techniques reduce the number of features in the data without losing important information, making the dataset more manageable and less computationally intensive to process.

Cloud Computing:

Cloud computing has revolutionised how businesses handle large datasets and complex computations, particularly in machine learning. As previously discussed, cloud computing offers a versatile and scalable solution to many challenges of handling big data and performing complex machine learning tasks. Here's how cloud computing can help overcome scalability issues:

  • Unlimited Storage

One of the primary benefits of cloud computing is virtually unlimited storage. It eliminates the need for physical storage systems, providing an expansive digital space to store vast data. As the dataset grows, it is easy to scale up the storage capacity without worrying about running out of space or purchasing additional hardware.

  • Powerful Computational Capabilities

Cloud computing platforms offer access to powerful processing capabilities. They allow leveraging high-performance computing resources on demand, including the latest CPUs, GPUs, and TPUs optimised for machine learning tasks. Complex machine learning models can be trained on large datasets more efficiently.

  • Scalable Infrastructure

Cloud computing provides a flexible and scalable infrastructure. Scaling up or down based on current needs makes it cost-effective. For instance, provisioning more resources is easy when training an ML model on a large dataset. Scaling down again once the training is complete saves costs.

  • Distributed Computing

Cloud platforms often support distributed computing frameworks like Apache Spark or Hadoop, allowing the distribution of computations across multiple machines. This capability enables processing larger datasets and performing complex calculations much faster than possible on a single machine.

  • Real-time Processing

Cloud computing facilitates real-time data processing, which is crucial for applications that require immediate insights, such as fraud detection or recommendation systems. With the power of cloud computing, data can be processed and analysed as it comes in, allowing machine learning models to learn and adapt in real-time.

  • Managed Machine Learning Platforms

Many cloud providers offer managed machine learning platforms. For instance, CUDO Compute handles much of the heavy lifting in training and deploying machine learning models. The platform provides tools for data preprocessing, algorithm selection, hyperparameter tuning, and model evaluation, simplifying the machine learning process.

There are challenges in implementing machine learning, and there are also solutions to overcome them. By understanding and addressing these challenges, ML practitioners can develop and deploy ML solutions that meet the expectations and requirements of their stakeholders.

About CUDO Compute

CUDO Compute is a fairer cloud computing platform for everyone. It provides access to distributed resources by leveraging underutilised computing globally on idle data centre hardware. It allows users to deploy virtual machines on the world’s first democratised cloud platform, finding the optimal resources in the ideal location at the best price.

CUDO Compute aims to democratise the public cloud by delivering a more sustainable economic, environmental, and societal model for computing by empowering businesses and individuals to monetise unused resources.

Our platform allows organisations and developers to deploy, run and scale based on demands without the constraints of centralised cloud environments. As a result, we realise significant availability, proximity and cost benefits for customers by simplifying their access to a broader pool of high-powered computing and distributed resources at the edge.

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