A helpful website for understanding gradient descent:
https://towardsdatascience.com/gradient-descent-explained-9b953fc0d2c

University of Michigan - Ann Arbor

A mathematical technique that modifies the parameters of a function to descend from a high value of a function to a low value, by looking at the derivatives of the function with respect to each of its parameters, and seeing which step, via which parameter, is the next best step to minimize the function. Applying gradient descent to the error function helps find weights that achieve lower and lower error values, making the model gradually more accurate.

Gradient Descent

Gradient Descent Reference

Gradient descent is used to properly update parameters in order to maximize the efficiency of linear regressions. Below is a step-by-step example of how to implement gradient descent.

Linear Regression and Gradient Descent

You can check your derivative computation to make sure that your implementation of back propagation is correct. You want to consider BOTH the right hand side and the left hand side derivatives. By taking a two sided derivative, you can numerically verify whether or not the function g of theta is a correct implementation of the derivative of f.

Numerical Approximation of Gradients

This is the technique used to check that our implementation is correct. There are different formulas for gradient checking and one of those is two-sided form:
          $$\frac {J(\theta + \epsilon) - J(\theta - \epsilon)} {2\epsilon}$$
Common choice for $\epsilon$ is $10^{-7}$. You shouldn't use gradient checking for the whole training data as it can be slow.

Gradient Checking

Assuming that the error function is $J(w)$ with one parameter $w$, to minimize the error, we can update the weight $w$ as follows.
$$w = w - \alpha * \frac{dJ(w)}{dw}$$
, where $\alpha$ is a learning rate, and $\frac{dJ(w)}{dw}$ is the derivative of $J(w)$ with respect to $w$.

If the error function has two or more parameters, for example, a weight $w$ and a bias $b$, we can update them one by one.
$$w = w - \alpha * \frac{\partial J(w,b)}{\partial w}$$
$$b = b - \alpha * \frac{\partial J(w,b)}{\partial b}$$
, where $\partial$ is a stylish cursive $d$, denoting the partial derivatives.

(Batch) Gradient Descent (Deep Learning Optimization Algorithm)

https://towardsdatascience.com/gradient-descent-explained-9b953fc0d2c

Gradient Descent Explained

Gradient Descent is an algorithm which is designed to find the optimal points, but these optimal points are not necessarily global. And yes if it happens that it diverges from a local location it may converge to another optimal point but its probability is not too much.

Consider the following "recliner chair" type of function(image below).

Obviously, this can be constructed so that there is a range in the middle where the gradient is the 0 vector, casuing the fail to find global optima. 


Why Gradient descent might fail?

DeepLearningAI. (2021, March 24). A Chat with Andrew on MLOps: From Model-centric to Data-centric AI. YouTube. https://www.youtube.com/watch?v=06-AZXmwHjo

A Chat with Andrew on MLOps: From Model-centric to Data-centric AI

Sagar, R. (2021, April 6). Big Data To Good Data: Andrew Ng Urges ML Community To Be More Data-Centric And Less Model-Centric. Analytics India Magazine. https://analyticsindiamag.com/big-data-to-good-data-andrew-ng-urges-ml-community-to-be-more-data-centric-and-less-model-centric/

Big Data to Good Data: Andrew Ng Urges ML Community To Be More Data-Centric and Less Model-Centric

Andrew Ng believes that for datasets with fewer than 10,000 data points, machine learning teams can make faster progress by focusing on improving data than improving code.

MLOps: Data-centric and Model-centric approaches

Objective function is the set of goal towards which the algorithm drives the solution, often this means to minimizing some error measure(cost function) or maximizing some probability function.

Because we are dealing with classification and prediction problems, the correct output is known. An error function (cost function) is defined, which captures the delta between the correct output and the actual output of the model, given the current model weights. We would need minimize the cost function, which is a type of an objective function. 

Objective Function

Points where the derivative of a function is 0 are known as critical points, or stationary points.

The derivative also has to be zero according to every possible directional derivative at that point in higher dimensional functions.

$$\triangledown f = 0$$

Critical Points

Optimization algorithms that use only the gradient, such as gradient descent,are called ﬁrst-order optimization algorithms.

First-order Optimization Algorithm

Optimization algorithms that also use the Hessian matrix, such as Newton’s method, are called second-order optimization algorithms

Second-order Optimization Algorithm

When the method of steepest descent is applied to a quadratic cost surface, it creates a zig-zag pattern that each line search direction is orthogonal to its previous line search direction.
Let $d_{t-1}$ be the previous search direction. At the minimum, we will find that:
$$\nabla _\theta f(\theta) \cdot d_{t-1} = 0$$
Then we know $d_t = \nabla _\theta f(\theta)$ is orthogonal to $d_{t-1}$

Method of Steepest Descent

Unlike first-order methods, second-order gradient methods use second-order derivatives. This improves optimization. 
- The most widely used second-order gradient method is the Newton method

Second-Order Gradient Methods

Gradient descent is an algorithm used to find the minimum value of a function. We will use the gradient descent algorithm to find the cost function. The idea of the algorithm is to randomly select a parameter combination at the beginning, calculate, and then find the next parameter combination that can reduce the cost function value the most, and continue to a minimum value.

https://machinelearningmastery.com/gradient-descent-for-machine-learning/

Gradient Descent Explanation

There are three variants of gradient descent, which differ in how much data we use to compute the gradient of the objective function: Batch Gradient Descent, Stochastic Gradient Descent and Mini-batch Gradient Descent. 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. 

Reference: https://ruder.io/optimizing-gradient-descent/

Gradient Descent Variants

Gradient descent methods use the slope of the surface. This will not necessarily point directly towards the extreme point. Local steepest direction may not be the same with the global optimum direction. 

Notes about gradient descent

Suppose you have built a neural network. You decide to initialize the weights and biases to be zero. Which of the following statements is true?

In a neural network with many time steps or layers, a gradient at the early layer is the product of all the terms from the later layers, which leads to an inherently unstable situation. Especially when the value of gradient has become so small, it no longer updates properly or is vanished eventually. Exploding gradient can be considered as the opposite of vanishing process. The updated weights using gradient descent become so large that they cause the whole network to become unstable, which leads to numerical overflow. 

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