What is Gradient Descent: A Comprehensive Guide for Beginners

What is Gradient Descent?

Gradient descent is a crucial optimization algorithm in machine learning that refines your neural network’s performance. The primary purpose of gradient descent is to minimize the neural network’s loss or error rate, resulting in more accurate predictions. It accomplishes this by adjusting the network’s parameters to lower the difference between its predictions and the expected values, also known as the loss.

The gradient descent algorithm uses calculus principles to fine-tune the initial parameter values and steer them in the direction that optimizes the network’s accuracy. Although you don’t need extensive calculus knowledge to grasp the concept of gradient descent, a basic understanding of gradients is helpful.

When implementing gradient descent in machine learning algorithms, consider the following elements:

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  • Parameters: These are the values your algorithm needs to optimize the neural network’s performance.
  • Gradient Descent Algorithm: The algorithm that performs optimization by iteratively tweaking the parameter values.
  • Optimization: The process of improving the neural network’s accuracy through minimizing the loss.
  • Alpha: A crucial factor that determines the step size during the iterative process of gradient descent.
  • Implement: This term refers to the process of applying gradient descent in your machine learning algorithm.

By using gradient descent, you will effectively optimize your neural network and improve its predictive accuracy.

What Are Gradients?

In the context of machine learning and deep learning, gradients play a crucial role in minimizing the error made by neural networks. Imagine a graph that shows the error of a neural network, with higher error values near the top and lower error values at the bottom. Your goal is to move from the top of the graph towards the bottom, where the error is the lowest.

Gradients quantify the relationship between the error and the weights of the neural network, represented as the slope of a function. The steepness of this slope indicates how quickly the model is learning. A steeper slope means the model is making larger reductions in its error and learning faster, while a slope of zero indicates that the model is not learning, as it is on a plateau.

To help the model move down the slope and reduce its error, you calculate the gradient. The gradient acts as a direction of movement for adjusting the neural network’s parameters. Picture a series of hills and valleys where your goal is to reach the lowest point of the valley, representing the lowest error. You can start at the top of the hill and take large steps downhill, confident that you are moving towards the lowest point of the valley.

However, as you get closer to the valley’s lowest point, your steps need to become smaller to avoid overshooting the actual lowest point. Similarly, in adjusting the weights of the neural network, the adjustments must become smaller over time to avoid moving away from the point of lowest error.

In this scenario, the gradient serves as a vector, providing instructions on the path to take and the size of the steps required. It informs you which direction to move in, which coefficients should be updated, and how much they should be updated. This allows your model to find the optimal solution efficiently and effectively, ultimately minimizing the error and improving its performance.

Estimating Gradients and Implementing Gradient Descent

While working with gradient descent, the first step is to initiate from a position with high loss. Through several iterations, you’ll take steps towards the lowest loss direction, aiming to discover the best weight configuration. To execute gradient descent effectively, it’s crucial to estimate gradients accurately.

In order to calculate the gradient, you need to know the cost or loss function. The cost function allows you to calculate the derivative. In calculus, the derivative refers to the slope of a function at a specific point. Thus, you’re primarily calculating the hill’s slope based on the loss function. You can determine the loss by running the coefficients through the loss function. Assuming the loss function is represented by “f”, the equation for calculating the loss is:

Loss = f(coefficient)

Next, calculate the derivative to identify the slope direction. Obtaining the loss’s derivative informs you of the uphill or downhill slope direction by providing the appropriate sign to adjust coefficients. Represent the appropriate direction as “delta”:

delta = derivative_function(loss)

Now that you know the direction toward the lowest loss point, you can update the coefficients in the neural network parameters and potentially reduce the loss. Update the coefficients based on previous coefficients minus the appropriate value change as determined by the direction (delta) and an argument controlling the magnitude of change (the step size). The argument that controls the update size is called the “learning rate”, denoted as “alpha”:

coefficient = coefficient - (alpha * delta)

Keep repeating this process until the network converges around the lowest loss point, which should be near zero.

Choosing the right learning rate (alpha) is crucial. It must be neither too small nor too large. As you approach the lowest loss point, steps must become smaller to avoid overshooting the true lowest loss point and ending up on the other side. If learning rate is too large, the network’s performance will bounce around the lowest loss point, overshooting it on either side, and it will never converge on the truly optimal weight configuration.

Conversely, if the learning rate is too small, the network might take an excessively long time to converge on the optimal weights.

Types of Gradient Descent

In your journey with gradient descent, you will encounter three main types, each with its own unique characteristics. The goal is to find the most suitable type for your particular problem and dataset.

Batch Gradient Descent processes the entire dataset before adjusting the model’s parameters. This makes it highly efficient in terms of computation, as fewer updates are needed. However, when dealing with a large number of training examples, this method becomes time-consuming.

Stochastic Gradient Descent takes a different approach. Each iteration involves processing just one training example and updating the parameters accordingly. As a result, convergence occurs more rapidly compared to Batch Gradient Descent. Although this method can take a long time if your dataset is extensive, in such cases, you might consider alternative types of gradient descent.

A compromise between the two aforementioned types is Mini-Batch Gradient Descent. This method divides the entire dataset into smaller batches, and after processing each batch, the error is calculated, and the parameters are updated. Mini-Batch Gradient Descent provides a balance between the computational efficiency of Batch Gradient Descent and the faster convergence of Stochastic Gradient Descent.

Regardless of which type you choose, these gradient descent variations have applications in various domains, such as linear regression, neural networks, and linear model learning algorithms. Selecting the most appropriate type will depend on factors such as dataset size and your model complexity. Remember to stay confident, knowledgeable, and clear in your understanding of gradient descent to make the best decision for your specific needs.

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