Machine Learning Unit 3 Semester 3 MSc IT Part 2 Mumbai University

TOPICS TO
BE
COVERED…
Linear Models: Least
Squares method,
Multivariate Linear
Regression, Regularized
Regression, Bias/Variance
Trade-off, Dimension
Reduction
Logistic Regression,
Gradient Descent
Perceptron, Support Vector
Machines, Soft Margin SVM,
Time Series Analysis,
Forecasting
PPT BY: MADHAV MISHRA 2

 What Is the Least Squares Method?
The "least squares" method is a form of mathematical regression analysis used to
determine the line of best fit for a set of data, providing a visual demonstration of
the relationship between the data points. Each point of data represents the
relationship between a known independent variable and an unknown dependent
variable.
 What Does the Least Squares Method Tell You?
The least squares method provides the overall rationale for the placement of the
line of best fit among the data points being studied. The most common application
of this method, which is sometimes referred to as "linear" or "ordinary", aims to
create a straight line that minimizes the sum of the squares of the errors that are
generated by the results of the associated equations, such as the squared residuals
resulting from differences in the observed value, and the value anticipated, based
on that model.
This method of regression analysis begins with a set of data points to be plotted on
an x- and y-axis graph.

 An analyst using the least squares method
will generate a line of best fit that explains
the potential relationship between
independent and dependent variables.
 In regression analysis, dependent variables
are illustrated on the vertical y-axis, while
independent variables are illustrated on the
horizontal x-axis. These designations will form
the equation for the line of best fit, which is
determined from the least squares method.
 When we fit a regression line to set of points,
we assume that there is some unknown linear
relationship between Y and X, and that for
every one-unit increase in X, Y increases by
some set amount on average.
 Our fitted regression line enables us to predict
the response, Y, for a given value of X.
 But for any specific observation, the actual
value of Y can deviate from the predicted
value. The deviations between the actual and
predicted values are called errors,
or residuals.

 Let’s look at the method of least squares from another
perspective. Imagine that you’ve plotted some data
using a scatterplot, and that you fit a line for the
mean of Y through the data. Let’s lock this line in
place, and attach springs between the data points and
the line.
 Some of the data points are further from the mean
line, so these springs are stretched more than others.
The springs that are stretched the furthest exert the
greatest force on the line.
 What if we unlock this mean line, and let it rotate
freely around the mean of Y? The forces on the
springs balance, rotating the line. The line rotates
until the overall force on the line is minimized.
 The are some cool physics at play, involving the
relationship between force and the energy needed to
pull a spring a given distance. It turns out that
minimizing the overall energy in the springs is
equivalent to fitting a regression line using the
method of least squares.

 Multivariate Regression is one of the simplest Machine Learning Algorithm. It
comes under the class of Supervised Learning Algorithms i.e, when we are
provided with training dataset.
 Multivariate Regression is a method used to measure the degree at which more
than one independent variable (predictors) and more than one dependent variable
(responses), are linearly related.
 The method is broadly used to predict the behavior of the response variables
associated to changes in the predictor variables, once a desired degree of relation
has been established.
 This is quite similar to the simple linear regression model we have discussed
previously, but with multiple independent variables contributing to the dependent
variable and hence multiple coefficients to determine and complex computation
due to the added variables.
 Jumping straight into the equation of multivariate linear regression,

 A researcher has collected data on three psychological variables, four academic
variables (standardized test scores), and the type of educational program the
student is in for 600 high school students. She is interested in how the set of
psychological variables is related to the academic variables and the type of
program the student is in.
 A doctor has collected data on cholesterol, blood pressure, and weight. She also
collected data on the eating habits of the subjects (e.g., how many ounces of red
meat, fish, dairy products, and chocolate consumed per week). She wants to
investigate the relationship between the three measures of health and eating
habits.
 A property dealer wants to set housing prices which are based various factors like
Size of house, No of bedrooms, Age of house, etc.
Note:
 Multiple Regression: The Multiple Regression model, relates more than one
predictor and one response.
 Multivariate Regression: The Multivariate Regression model, relates more than
one predictor and more than one response.

 Regularization is used as a solution to get rid out of the overfitting problem in
multivariate regression, but it can be used in both univariate and multivariate
regression.
 In general, regularization means to make things regular or acceptable.
 In the context of machine learning, regularization is the process which regularizes or
shrinks the coefficients towards zero and in simple words, regularization discourages
learning a more complex or flexible model, to prevent overfitting.
 How Does Regularization Work?
The basic idea is to penalize the complex models i.e. adding a complexity term that
would give a bigger loss for complex models. To understand it, let’s consider a simple
relation for linear regression. Mathematically, it is stated as below:
Y≈ W_0+ W_1 X_1+ W_2 X_(2 )+⋯+W_P X_P
Where Y is the value to be predicted,
X_1,X_(2 ),〖…,X〗_P , are the features deciding the value of Y.
W_1,W_(2 ),〖…,W〗_P , are the weights attached to the features X_1,X_(2 ),〖…,X〗_P
respectively.
W_0 represents the bias. PPT BY: MADHAV
MISHRA 10

 Regularization keeps all the features in Multivariate regression but reduces
magnitude values of parameters θj
( θ mean weight of your function )
 Cost Function : It is a measure that measures the performance of a ML model for
a given data.
It qualifies error between predicted and expected values present in the form of
single real number.
Depending upon the problem the cost function can be formed in many different
ways.
 Now, in order to fit a model that accurately predicts the value of Y, we require a
loss function and optimized parameters i.e. bias and weights.
 The loss function generally used for linear regression is called the residual sum of
squares (RSS). According to the above stated linear regression relation, it can be
given as:
/

Regularization Techniques
 There are two main regularization techniques, namely Ridge Regression and
Lasso Regression. They both differ in the way they assign a penalty to the
coefficients.
 They are also known as L1 (Lasso Regression) and L2 (Ridge Regression)
Ridge Regression (L2)
 Ridge Regression is a technique which comes into picture when the data suffers
from Multicollinearity (which simple means that independent variables are highly
correlated).
 In Multicollinearity concept, even though the least square estimates are unbiased,
their variance are large which in turn return results in the deviation of the
observed value far from the true values.
VALUE OBSERVED VALUE far from TRUE VALUE
Observed values – predicted value
True value – actual value

 By adding a degree of bias to the regression estimates, ridge regression is able to
reduce the standard error.
 So Linear Regression:
Y = a + b * X
 By adding an error term (degree of bias)
Y = a + b * X + e
(error term – it is the value needed to correct prediction error between the observed
& predicted value)
Y = a+b1X1+b2X2+……+ e
 In linear equation, it is possible to solve prediction error into sub components.
1st component – due to bias
2nd component – due to variance
Prediction error mostly occurs due to any one of these two or both component.

 Ridge Regression solves the multicollinearity problem through shrinkage (lambda).
 Here we have two components, First one is least square term & another is lambda of
the summation of β2 (beta square)
 β is coefficient , is added to the least square term in order to shrink the parameter to
have a very low variance
 Important Terms:
It shrinks the value of coefficient but does not reach zero.
This regularization is called L2 Regularization.

 It stands for least absolute shrinkage and selection operator.
 Lasso Regression is a type of linear regression that uses shrinkage, where data
values are shrunk towards a central point, like the mean.
 The lasso procedure encourages simple, sparse models.
 This is well suited for models showing high level of multicollinearity or when you
want to automate certain parts of models selection, like variable selection or
elimination.
 Lasso was introduced in order to improve the prediction accuracy and
interpretability of regression models.
 This is done by taking only a subset of provided covariates for use in the final
model rather than using all of them.
 Lasso is an alternative to avoid many problems of overfitting in model.

 Lasso regression performs L1 regularization which adds a factor of sum of absolute
values of coefficients in the optimization objective.
 Where RSS stands for Least Squares Objective which is nothing but the linear
regression objective without regularization and λ is the turning factor that controls the
amount of regularization. The bias will increase with the increasing value of λ and the
variance will decrease as the amount of shrinkage (λ) increases.
 Here the turning factor λ controls the strength of penalty, that is
When λ = 0: We get same coefficients as simple linear regression
When λ = ∞: All coefficients are zero
When 0 < λ < ∞: We get coefficients between 0 and that of simple linear regression

 Why Do You Need to Apply a Regularization Technique?
Often, the linear regression model comprising of a large number of features suffers
from some of the following:
Overfitting: Overfitting results in the model failing to generalize on the unseen
dataset.
Multicollinearity: Model suffering from multicollinearity effect.
Computationally Intensive: A model becomes computationally intensive.
 When Do You Need to Apply Regularization Techniques?
Once the regression model is built and one of the following symptoms happen, you
could apply one of the regularization techniques.
Model lack of generalization: Model found with higher accuracy fails to generalize
on unseen or new data.
Model instability: Different regression models can be created with different
accuracies. It becomes difficult to select one of them.

 Bias & variance are ways of measuring the difference between your prediction and
actual outcome.
 Bias is called as error. It is useful to quantify how much on an average are the
predicted values different from the actual values.
(gap between your predicted value and the actual value or outcome)
 Variance helps to quantify how are the prediction made on some observation
different from each other.
(when your predicted values are scattered all over the places)
 A high bias error in a model results to have a under performing model, which
keeps on missing important trends.
 A high variance model will overfit on your training population and perform badly
on any observation beyond training

 High Bias / High Variance - Consistently
wrong is an inconsistent way.
 High Bias / Low Variance - Consistently
wrong.
 Low Bias / High Variance - One bulls
target.
 High Bias can led to missing of the
relevance data or feature needed for the
target value in other words leads to
underfitting.
 High Variance can lead to generation of
random noise in the training data and
can deviate the output that leads to
overfitting.
 In order to have perfect fit in the model,
the bias & variance should be balanced.

 The following bulls-eye diagram explains the tradeoff better:
 The center i.e. the bull’s eye is the model result we want to
achieve that perfectly predicts all the values correctly.
 As we move away from the bull’s eye, our model starts to make
more and more wrong predictions.
 A model with low bias and high variance predicts points that
are around the center generally, but pretty far away from each
other.
 A model with high bias and low variance is pretty far away
from the bull’s eye, but since the variance is low, the predicted
points are closer to each other.
 we learned that an ideal model would be one where both the
bias error and the variance error are low. However, we should
always aim for a model where the model score for the training
data is as close as possible to the model score for the testing
data.
 That’s where we figured out how to choose a model that is not
too complex (High variance and low bias) which would lead to
overfitting and nor too simple(High Bias and low variance)
which would lead to underfitting.
 Bias and Variance plays an important role in deciding which
predictive model to use. PPT BY: MADHAV
MISHRA
20

 In ML during classification , we get many cases when we cross “N” no of
dimensions or features/ parameters/ attributes
 The motivation behind dimensionality reduction is to cut down (remove/
eliminate) unwanted dimensions or features which will finally classify the dataset
into correct class.
 Dimensionality reduction can also be referred as the process of converting a set of
data having base dimensions into data with lesser dimension ensuring that it
provides the same or similar information.
 Let’s understand with example, if we have say 2 dimensions X1 and X2.
 Which tells us the measurements of several objects in cm(X1) & inches(X2).
 Now if we use both these dimensions in machine learning, they will convey similar
information & introduce a lot of noise in system. So better to use one dimension in
place of two.
 We then convert the dimension of data 2D (from X1 & X2) to 1D(Z1).

 Process of Dimensionality reduction can be divided into mainly 2 types.
 Feature Selection & Feature Extraction.
 Methods: Dimensionality reduction techniques
1. Missing Value Ratio: dataset has Attributes/Columns has many missing values
which is not useful feature.
2. Low Variance: We compare feature to feature and see the value and difference. The
value and difference that are minimum or has minimum difference is been removed.
3. High Correlation Filter: Here if a one feature is contributing an information and at
the same time another feature is contributing the same information then we see
high correlation between both the feature, since the information derived by both the
features are same we tend to remove one feature amongst both.
4. PCA: Also known as principal component analysis, also they are orthogonal in
nature. It is like a tool that can be used to reduce a large set of variables to a small
set that still contains most of the information that the large set had. Mathematical
procedures that transforms a number of possible or correlated variables into a
smaller number of uncorrelated variable called as PCA.
5. Back feature elimination: here we have number of feature , say (n) feature so once
we train the model by using back feature elimination method we tend to train it as
(n) train, (n-1) train, (n-2) train. We train all the feature and check the error rate for
each feature. If the error rate is less we keep the feature for model building going
ahead, but if the error rate is increasing then we remove the feature.
6. Forward feature elimination: here we first create a empty list of feature, and then
add feature that has list that has less error using the mechanism (n) train,
(n+1)train, (n+2) train and so on.

 Logistic Regression is used for a different class of problems known as
classification problem.
 Here the aim is to predict the group which any current object under observation
belongs to.
 It gives you a discrete binary outcome between 0 & 1.
 A simple example would be whether a person will vote or not in a upcoming
elections.
 How does it works?
Logistic regression measures the relationship between the dependent variable
(our label, what we want to predict) and the one or more independent variables
(our features), by estimating probabilities using its underlying logistic function.
It uses sigmoid function which is given as

 The sigmoid function is an S- Shaped Curve that can take any real valued number
and map it into a value between the range of 0 to 1, but never exactly at those
limits.
 Making Predictions:
These probabilities must then be transformed into binary values in order to actually
make a prediction.
This is the task of the logistic function also called a sigmoid function.
This values between 0 & 1 will then be transformed into either 0 or 1 using a
threshold classifier.
Logistic Vs Linear:
Logistic regression gives you a discrete outcome but Linear regression gives you a
continuous outcome

 Time Series Analysis also known as TSA.
 TSA consist of method used to analyse various data facts or statistics from various
characteristics of the data.
 TSA used for continuous data, for example economic growth of an organization,
share price, sales temperature, weather etc.
 TSA model has time ‘t’ as an independent variable & the target is a dependent
variable denoted by Yt
 The output from the time series model is a a predicted value of y at the given time
t.
 Time Series is the process of recording of the data at regular interval of time.
 TSA Components:
TRENDS , CYCLES, SEASONALITY

 Trends:
Considered to behaviour of the feature at a particular amount of time, it can be
categorized as increasing trend, decreasing trend or constant trend.
 Seasonality:
Pattern which repeats at the constant frequency. Example here the demand for the
umbrella will be at peak in the rainy season.
 Cycles:
They are the type of seasonality pattern but it doesn’t repeat at regular frequency.
Cycles can be generally considered as the task completion time.
Example: Iterative model of S/W Engineering, every iteration can have different
time requirement. But here every task has to undergo all stage in a single iteration.
Most widely used time series analysis is Autoregressive Moving Average (ARMA),
Which has two parts in them (AR) Autoregressive and (MA) Moving Average.

 The Process of making prediction of future based on the present and the past data
most commonly by using analysis of trends is called as forecasting.
 Steps for forecasting:
1. Define the goal or business object.
2. Get the required data.
3. Explores & Visualize the series.
4. Pre- process the data.
5. Partition the series.
6. Apply suitable forecasting model(ARMA Model)
7. Evaluate & compare the performance of the system.
8. Implement the final forecasting system.

 What is a neural network?
A neural network is formed when a collection of nodes or neurons are interlinked
through synaptic connections.
There are three layers in every artificial neural network – input layer, hidden layer,
and output layer.
The input layer that is formed from a collection of several nodes or neurons receives
inputs.
Every neuron in the network has a function, and every connection has a weight
value associated with it.
Inputs then move from the input layer to layer made from a separate set of neurons
– the hidden layer. The output layer gives the final outputs.

 Perceptron
 A perceptron is a neural network unit (an artificial neuron)
that does certain computations to detect features or
business intelligence in the input data.
 A perceptron, a neuron’s computational prototype, is
categorized as the simplest form of a neural network.
 Frank Rosenblatt invented the perceptron at the Cornell
Aeronautical Laboratory in 1957.
 A perceptron has one or more than one inputs, a process,
and only one output.
 The concept of perceptron has a critical role in machine
learning.
 It is used as an algorithm or a linear classifier to facilitate
supervised learning of binary classifiers.
 Supervised learning is amongst the most researched of
learning problems.
 A supervised learning sample always consists of an input
and a correct/explicit output.
 The objective of this learning problem is to use data with
correct labels for making predictions on future data, for
training a model.
 Some of the common problems of supervised learning
include classification to predict class labels.

 A linear classifier that the perceptron is categorized as is a classification
algorithm, which relies on a linear predictor function to make predictions.
 Its predictions are based on a combination that includes weights and feature
vector.
 The linear classifier suggests two categories for the classification of training data.
 This means, if classification is done for two categories, then the entire training
data will fall under these two categories.
 The perceptron algorithm, in its most basic form, finds its use in the binary
classification of data.
 Perceptron takes its name from the basic unit of a neuron, which also goes by the
same name.

 There are two types of Perceptrons:
Single layer and Multilayer.
 Single layer Perceptrons can learn only
linearly separable patterns.
 Multilayer Perceptrons or feedforward
neural networks with two or more layers
have the greater processing power.
 The Perceptron algorithm learns the
weights for the input signals in order to
draw a linear decision boundary.
 This enables you to distinguish between
the two linearly separable classes +1 and
-1.

 Perceptron Learning Rule
states that the algorithm
would automatically learn
the optimal weight
coefficients.
 The input features are then
multiplied with these
weights to determine if a
neuron fires or not.
 The Perceptron receives
multiple input signals, and if
the sum of the input signals
exceeds a certain threshold,
it either outputs a signal or
does not return an output.
 In the context of supervised
learning and classification,
this can then be used to
predict the class of a sample.

 Perceptron is a function that maps its input “x,” which is multiplied with the learned
weight coefficient; an output value ”f(x)”is generated.
 In the equation given above:
“w” = vector of real-valued weights
“b” = bias (an element that adjusts the boundary away from origin without any
dependence on the input value)
“x” = vector of input x values
 “m” = number of inputs to the Perceptron
 The output can be represented as “1” or “0”. It can also be represented as “1” or “-1”
depending on which activation function is used.

 A Perceptron accepts inputs, moderates them with certain weight values, then
applies the transformation function to output the final result.
 The above below shows a Perceptron with a Boolean output.
 A Boolean output is based on inputs such as salaried, married, age, past credit
profile, etc. It has only two values: Yes and No or True and False.
 The summation function “∑” multiplies all inputs of “x” by weights “w” and then
adds them up as follows:

 The activation function applies a step rule (convert the numerical output into +1
or -1) to check if the output of the weighting function is greater than zero or not.
 Step function gets triggered above a certain value of the neuron output; else it
outputs zero.
 Sign Function outputs +1 or -1 depending on whether neuron output is greater
than zero or not.
 Sigmoid is the S-curve and outputs a value between 0 and 1.

 Steps to perform a perceptron learning algorithm
1. Feed the features of the model that is required to be trained as input in the first
layer.
2. All weights and inputs will be multiplied – the multiplied result of each weight
and input will be added up
3. The Bias value will be added to shift the output function
4. This value will be presented to the activation function (the type of activation
function will depend on the need)
5. The value received after the last step is the output value.

 Support Vector Machine” (SVM) is a
supervised machine learning
algorithm which can be used for both
classification or regression challenges.
 However, it is mostly used in
classification problems.
 In the SVM algorithm, we plot each data
item as a point in n-dimensional space
(where n is number of features you have)
with the value of each feature being the
value of a particular coordinate.
 Then, we perform classification by
finding the hyper-plane that
differentiates the two classes very well.
 Support Vectors are simply the co-
ordinates of individual observation.
 The SVM classifier is a frontier
which best segregates the two classes
(hyper-plane/ line).

 Hyperparameters of the Support Vector Machine (SVM)
Algorithm
There are a few important parameters of SVM that you
should be aware of before proceeding further:
Kernel: A kernel helps us find a hyperplane in the higher
dimensional space without increasing the computational
cost. Usually, the computational cost will increase if the
dimension of the data increases. This increase in dimension
is required when we are unable to find a separating
hyperplane in a given dimension and are required to move
in a higher dimension(mentioned in the picture).
Hyperplane: This is basically a separating line between two
data classes in SVM. But in Support Vector Regression, this
is the line that will be used to predict the continuous output
Decision Boundary: A decision boundary can be thought of
as a demarcation line (for simplification) on one side of
which lie positive examples and on the other side lie the
negative examples. On this very line, the examples may be
classified as either positive or negative. This same concept of
SVM will be applied in Support Vector Regression as well

 How does it work?
Identify the right hyper-plane (Scenario-1): Here, we have three hyper-planes (A, B
and C). Now, identify the right hyper-plane to classify star and circle. You need to
remember a thumb rule to identify the right hyper-plane: “Select the hyper-plane
which segregates the two classes better”. In this scenario, hyper-plane “B”
has excellently performed this job.

Identify the right hyper-plane (Scenario-2): Here, we have three hyper-planes (A, B
and C) and all are segregating the classes well. Now, How can we identify the right
hyper-plane?
Here, maximizing the distances between nearest data point (either class) and
hyper-plane will help us to decide the right hyper-plane. This distance is called
as Margin.
Above, you can see that the margin for hyper-plane C is high as compared to both A
and B. Hence, we name the right hyper-plane as C. Another lightning reason for
selecting the hyper-plane with higher margin is robustness. If we select a hyper-
plane having low margin then there is high chance of miss-classification.

 Identify the right hyper-plane (Scenario-3):Hint: Use the rules as discussed in
previous section to identify the right hyper-plane
 Some of you may have selected the hyper-plane B as it has higher margin
compared to A. But, here is the catch, SVM selects the hyper-plane which
classifies the classes accurately prior to maximizing margin. Here, hyper-plane B
has a classification error and A has classified all correctly. Therefore, the right
hyper-plane is A.

Can we classify two classes
(Scenario-4)?:
Below, I am unable to segregate the
two classes using a straight line, as one
of the stars lies in the territory of
other(circle) class as an outlier.
As I have already mentioned, one star
at other end is like an outlier for star
class. The SVM algorithm has a feature
to ignore outliers and find the hyper-
plane that has the maximum margin.
Hence, we can say, SVM classification is
robust to outliers.

Find the hyper-plane to segregate to classes
(Scenario-5):
In the scenario below, we can’t have linear hyper-
plane between the two classes, so how does SVM
classify these two classes? Till now, we have only
looked at the linear hyper-plane.
SVM can solve this problem. Easily! It solves this
problem by introducing additional feature. Here,
we will add a new feature z=x^2+y^2. Now, let’s
plot the data points on axis x and z.
In the final plot, points to consider are:
 All values for z would be positive always because
z is the squared sum of both x and y
 In the original plot, red circles appear close to
the origin of x and y axes, leading to lower value
of z and star relatively away from the origin
result to higher value of z.

 What Soft Margin does is:
The soft margin SVM gives more flexibility by
allowing some of the training points to be
misclassified.
It tolerates a few dots to get misclassified
It tries to balance the trade-off between finding
a line that maximizes the margin and
minimizes the misclassification.
 Two types of misclassifications can happen:
1. The dot is on the wrong side of the decision
boundary but on the correct side/ on the margin
(shown in left)
2. The dot is on the wrong side of the decision
boundary and on the wrong side of the margin
(shown in right)
 Either case, the support vector machine
tolerates those dots to be misclassified when it
tries to find the linear decision boundary.

 What is Gradient Descent?
It is an optimization algorithm used to find the
values of parameters .i.e. coefficients of a function
(f) that minimizes a cost function (cost).
It is defined as First-order iterative optimization
algorithm for finding the minimum of a loss
function.
It is also one of the most popular and widely used
optimization algorithm.
Given a machine learning model with parameters
(weights and biases) and a cost function to see
how good a model is, our learning problem reduces
to find a good set of weights for our model which
minimizes the cost function.
(Cost Function : It is a measure that measures the
performance of a ML model for a given data.)
(Learning Problem: It is a decision problem that
needs to be modelled from data)

 Gradient descent is an iterative method.
 So we start some value for our model parameters (weights and biases), and
improve them slowly.
 To improve a set of weights, we try to get a sense in terms of the value of the cost
function for weights similar to the current weights (by calculating the gradient)
and move in the direction in which the cost function reduces (decreases or is
negative).
 So standing on an iterative methodology we tend to repeat this step thousands of
times.
 Hence by this we’ll minimize our cost function by the above explained iterative
process.
 Let’s try to know the Equations and formulas in it:
 Gradient descent is used to minimize a cost function J(w) which is parameterized
alongside by a model parameters w. The gradient (or derivative) shows us the
incline or slope of the cost function. So to minimize the cost function, we move in
the direction opposite to the gradient.
 Let G be the gradient of the cost function with respect to the parameters at a
particular value w of the weight vector. That is,

 Thereafter, the gradient descent step is given by
 η = learning rate that determines the size of the steps which is taken to reach a
minimum
 Note : Here we just need to be careful about this parameter i.e. high values of η
may go past the global minimum & then the low value will reach minimum slowly.

 Steps to perform Gradient Descent:
Step 1. Initialize the weights w randomly
Step 2. Calculate the gradients G of cost function w.r.t parameters
Step 3. Update the weights by an amount proportional to G, i.e. w = w -ηG
Step 4. Repeat until J(w) stops reducing or other pre-defined termination criteria is
met

 Let’s think little & understand more with below
example
 Imagine you’re blind folded riding the car, and your
objective is to reach the lowest altitude.
 One of the simplest strategies you can use, is to the
tyre wheel on the ground will move only in the
downward direction on the slope downwards, and
considering that it is taking a step in the direction
where the ground is descending the fastest (the tyre
wheel moves fast on that direction).
 If you keep repeating this process, you might slide up
& down, and land up somewhere in the minimum side
of the valley.
 The riding car is analogous to the cost function.
 Minimizing the cost function is analogous on trying to
reach the lower altitudes.
 Feeling the slope by the cars wheel around is
analogous to calculating the gradient, and taking a
step and moving the car on slope is analogous to one
iteration for the parameter.

 Finally let’s see the multiple variants of Gradient Descent
 It consists of multiple variants which are used depending on the amount of data
which is being used to calculate the gradient.
 Reason for this variation is the computational efficiency of the models because
they can have many (million) data points in a datasets.
 So in this calculating entire dataset is very expensive. So it is divided as the
 Batch gradient descent, Stochastic gradient descent & Mini-Batch Gradient
descent.
 Batch gradient descent
It computes the gradient of the cost function w.r.t to parameter w for entire training
data.
As we need to calculate the gradients for the entire dataset to perform one basic
parameter update.
Hence batch gradient descent can be very slow as of it computational efficiency.

 Stochastic gradient descent
Here it computes the gradient for each training sample (xi) i.e. a single training
data point is used for each update.
 Mini-Batch gradient descent
Here we calculate the gradient for each small mini-batch of training data.
We perform it as:
First divide the training data into small batches (say M samples / batch) then we
perform one update per mini-batch. M is usually in the range 30–500, depending on
the problem.
 Amongst all of these mini-batch & Stochastic Gradient Descent are most popular.
 Here mini-batch is used for computing infrastructure which can be compliers or
CPUs

Machine Learning Unit 3 Semester 3 MSc IT Part 2 Mumbai University

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Machine Learning Unit 3 Semester 3 MSc IT Part 2 Mumbai University