Gradient descent Algorithm: In-Depth explanation

Kajal Pawar

2 years ago

In some of my previous articles, I have explained about the activation functions and loss functions used in machine learning/deep learning. And also written an article on optimizers and its types. I recommend you to once go through it for more better understanding.
In this article, I will give you an in-depth explanation of Gradient descent optimizers and its different types.
So, let’s start at…

What is an optimizer in Machine Learning/Deep Learning?

In previous articles, we saw how to deal with loss functions, which is a mathematical way of measuring how wrong our predictions are.
During the training process, we tweak and change the parameters (weights) of our model to try and minimize that loss function, and make our predictions as correct and optimized as possible.
But you may be thinking that how exactly do we do that? How do we change the parameters of our model, by how much, and when? This all questions are very important which surely affects our model performance.
Now, where the optimizers come into the picture. Optimizers try to combined together the loss function and model parameters by updating the model in response to the output of the loss function. In simpler terms, we can say that the optimizers shape and mould our model into its most accurate possible form by dabbling with the weights. The loss function act as a guide to the terrain, telling the optimizer when it’s moving in the right or wrong direction.
Let’s take a simple example and try to understand what simply happening.
Imagine, one day you and your friends went for trekking. All of you reached on the top of a mountain. As you are tired and want some rest, you told your friends to move forward and get down you will be joining them after taking some rest. While you trying to get down a mountain with a blindfold on. It’s impossible to know which direction to go in, but there’s one thing you can know: if you will be going down (making progress) or going up (losing progress). Eventually, if you keep taking steps that lead you downwards, you’ll reach the base.
Similarly, it’s impossible for us to know what our model’s weights should be right from start. But with some trial and error based on the loss function (whether you descending), you can end up getting there eventually.
Now as we know any discussion about optimizers needs to begin with the most popular one, and which is known as Gradient Descent. This algorithm is used across all types of Machine Learning and Deep Learning problems which are to be optimized. It’s fast, robust, and flexible and good performance. 

Gradient Descent

Gradient descent is one of the types of an optimization algorithm used to minimize some loss function by iteratively moving in the direction of steepest descent as defined by the negative of the gradient. In machine learning, we use gradient descent to update the parameters of our model. These parameters are nothing but they refer to coefficients in Linear Regression in machine learning and weights in neural networks in deep learning.
Let’s see how it works:
1. It tries to calculate what a small change in each individual weight would do to the loss function (i.e. which direction should the hiker walk-in)
2. Then it adjusts each individual weight based on its gradient (i.e. take a small step in the determined direction)
3. It keeps iterating step 1 and step 2 until the loss function gets as low as possible and get the best model.
Note: The ultimate aim of this algorithm is to reach to the global minima and do not get stuck at the local minima.
So, you might be thinking what is Gradient and descent are in gradient descent algorithm?
As of now, you may know, Gradients are nothing but partial derivatives wrt weights and loss and are a measure of change. And Descent means in which direction we should move to achieve the global minima. They connect the loss function and the weights; they tell us what specific operation we should perform to our weights – add 6, subtract .06, or anything else which helps us to lower the output of the loss function and thereby make our model more accurate.
There are some other elements such as cost function, learning rate, etc. which play an important role that makeup Gradient Descent and also generalize to other optimizers.
What is Cost Function?
The main point for learning neural networks is to define a cost function which is also known as a loss function. Cost/Loss functions measures how well the network predicts outputs on the test set. The goal is to then find a set of weights and biases values that minimizes the cost/loss. One common function that is often used is the mean squared error, which measures the difference between the actual value of y and the estimated value of y (the prediction).
The equation of the below regression line is hθ(x) = θ + θ1x, which has only two parameters: weight (θ1) and bias (θ0).
What is Learning rate?
Learning rate is nothing but the size of the steps. Its plays a very important role in optimizing our model. With a high value of learning rate, we can capture more ground in each step, but we may risk overshooting the minima point as the slope of the hill is constantly changing. On the other hand, with a very low learning rate, we can move in the direction of the negative gradient as we are recalculating it so frequently.
A low learning rate is more precise, but it’s a time-consuming, so it will take us a very long time to achieve the global minima point. (lowest point) and sometimes it also gets stuck at the local minima.
So, choosing the correct value of the learning rate plays an important role in our model performance.
Let’s take an example and try to understand how the gradient descent works?
Let’s try to predict the price of a new how house from the given housing data:
Below is the given historical data:
Task: In this example our task is to predict the price of a new house based on the size of the house.
Let’s plot the given historical housing data:
From the above figure, we can see it follows a linear trend as the size of the house increases, the price is also increasing.
Next, let’s try to use a simple linear regression model, where we try to fit a line on the given historical data and predict the price of a new house (Ypred) based on its size (X).
From the above figure, we can see that the red line gives the predicted house price (i.e., Ypred) given house size(X).
So, Ypred can be given as:
Ypred = a+bX
Now, the blue line represents the actual prices from the historical data i.e., Yactual.
The difference between Yactual and Ypred which is represented by the yellow dashed lines is called prediction error (error) E.
So now, our aim is to find a line with optimal values of a, b which is known as coefficients/weights that best fits the historical data by reducing the prediction error/ error and improving prediction accuracy.
Here, our goal is to find optimal values of a, b that can minimizes the error between actual and predicted values of house price
(NOTE: 1/2 is used for mathematical convenience since it helps us in calculating gradients in calculus easily)
Sum of Squared Errors (SSE) = ½ Sum (Actual House Price – Predicted House Price)2
SSE = ½ Sum (Y – Ypred)2
Note: There are other types of measures of Error. Here, we used SSE which is just one of them).
This is where the Gradient Descent comes into the picture. Gradient descent is an optimization algorithm that finds the optimal values of weights a, b that reduces the prediction error.
Let’s now try to understand the Gradient Descent algorithm with an example:
Below are the steps involved in the gradient descent algorithm are mentioned.
Step 1: First, Initialize the weights (a & b) with random values and calculate Error (SSE).
Step 2: Calculate the gradient i.e. change in SSE when the weights (a & b) are changed by a very small value from their original randomly initialized value. This helps us move the values of a & b in the direction in which SSE can be minimized.
Step 3: Now, we adjust the weights with the gradients to reach the optimal values so that SSE can be minimized.
Step 4: Use these new weights for prediction and then calculate the new SSE.
Step 5: Finally, we repeat steps 2 and 3 till further adjustments to weights doesn’t significantly reduce the Error.
Let’s now go through each of the steps in detail.
But before that, we have to standardize the data as it will make the optimization process faster and convenient.
Step 1: To fit a line Ypred = a + b X, we start off with random values of a and b and then calculate the prediction error (SSE) accordingly as shown below.
Step 2: In our second step we calculate the error gradient w.r.t the weights as shown below.
∂SSE/∂a = – (Y-Ypred)
∂SSE/∂b = – (Y-Ypred) X
Here, SSE = ½ (Y-Ypred)2 = ½ (Y- (a+bX))2
Here, ∂SSE/∂a and ∂SSE/∂b are the gradients and which give the direction of the movement of a, b w.r.t to SSE.
Step 3: Adjust the weights with the gradients to reach the optimal values where SSE is minimum. 
Now, we need to update the random values of a, b so that we can move in the direction of optimal a, b where total SEE is minimum.
Update rules:
• a – ∂SSE/∂a
• b – ∂SSE/∂b
So, update rules:
1. New a = a – lr * ∂SSE/∂a = 0.45-0.01*3.300 = 0.42
2. New b = b – lr * ∂SSE/∂b= 0.75-0.01*1.545 = 0.73
Here, lr is the learning rate = 0.01, which is the pace of adjustment to the weights. I already explained you about learning rate earlier.
Note: These values are randomly taken to explain you the concepts. Don’t consider them as a fixed one.
Step 4: Now, we use these new a and b values for our prediction and then use it to calculate the new Total SSE.
Now we can observe from the above table, with the new prediction, the total SSE has gone down (0.677 to 0.553). That implies our prediction accuracy has improved and model will show better performance.
Step 5: Finally, in our last step we repeat step 3 and 4 till the time further adjustments to a, b doesn’t show significantly reduce in the error.
At that time, we have arrived at the optimal value of a, b with the highest prediction accuracy.
This is how the Gradient Descent Algorithm works behind the scene. This optimization algorithm and different variants form the core of many machine learning and Deep Learning.
Note: We don’t have to do all this manually. There are many built-in functions available which performs all this for us. Only we have to call them.
Types of Gradient descent optimizers
There are many different types but mainly these three variants of gradient descent are common, which differ in how much data we use to compute the gradient of the objective function. Depending on the amount of data, we make a trade-off between the accuracy of the parameter update and the time it takes to perform an update.
1. Batch gradient descent/ Vanilla gradient descent
2. SGD (Stochastic gradient descent)
3. Mini-batch gradient descent
Let’s see how they differ from each other’s.
Batch gradient descent/ Vanilla gradient descent
Gradient update rule: BGD uses the data of the entire training set to calculate the gradient of the cost function to the parameters:
Because this method calculates the gradient for the entire data set in one update, the calculation is very slow, it will be very tricky to encounter a large number of data sets, and you cannot invest in new data to update the model in real time.
We will define an iteration number epoch in advance, first calculate the gradient vector params_grad, and then update the parameter params along the direction of the gradient. The learning rate determines how big we take each step.
Batch gradient descent can converge to a global minimum for convex functions and to a local minimum for non-convex functions.

SGD (Stochastic gradient descent)

Gradient update rule: Compared with BGD's calculation of gradients with all data at one time, SGD updates the gradient of each sample with each update.
x += - learning_rate * dx
where x is a parameter, dx is the gradient and learning rate is constant
For large data sets, there may be similar samples, so BGD calculates the gradient. There will be redundancy, and SGD is updated only once, there is no redundancy, it is faster, and new samples can be added.

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