# Linear Regression and General ML Questions

Question 1) What is linear regression?

In simple terms, linear regression is a method of finding the best straight line fitting to the given data, i.e. finding the best linear relationship between the independent and dependent variables.

In technical terms, linear regression is a machine learning algorithm that finds the best linear-fit relationship on any given data, between independent and dependent variables. It is mostly done by the Sum of Squared Residuals Method.

Question 2) State the assumptions in a linear regression model.

There are three main assumptions in a linear regression model:

1. Assumption about the form of the model: It is assumed that there is a linear relationship between the dependent and independent variables. It is known as the ‘*linearity assumption*’.

2. Assumptions about the residuals:

1. *Normality assumption*: It is assumed that the error terms, ε(i), are normally distributed.

2. *Zero mean assumption*: It is assumed that the residuals have a mean value of zero.

3. *Constant variance assumption*: It is assumed that the residual terms have the same (but unknown) variance, σ2. This assumption is also known as the assumption of homogeneity or homoscedasticity.

4. *Independent error assumption*: It is assumed that the residual terms are independent of each other, i.e. their pair-wise covariance is zero.

1. The independent variables are measured without error.

2. The independent variables are linearly independent of each other, i.e. there is no multicollinearity in the data.

3. Assumptions about the estimators:

Explanation:

1. This is self-explanatory.

2. If the residuals are not normally distributed, their randomness is lost, which implies that the model is not able to explain the relation in the data. Also, the mean of the residuals should be zero. Y(i)i = β0+ β1x(i) + ε(i)

This is the assumed linear model, where ε is the residual term. E(Y) = E(β0 + β1x(i) + ε(i)) = E(β0 + β1x(i)) + E(ε(i)) If the expectation(mean) of residuals, E(ε(i)), is zero, the expectations of the target variable and the model become the same, which is one of the targets of the model. The residuals (also known as error terms) should be independent. This means that there is no correlation between the residuals and the predicted values, or among the residuals themselves. If some correlation is present, it implies that there is some relation that the regression model is not able to identify.

3. If the independent variables are not linearly independent of each other, the uniqueness of the least squares solution (or normal equation solution) is lost.

Question 3) What is feature engineering? How do you apply it in the process of modelling?

Feature engineering is the process of transforming raw data into features that better represent the underlying problem to the predictive models, resulting in improved model accuracy on unseen data.

In layman terms, feature engineering means the development of new features that may help you understand and model the problem in a better way. Feature engineering is essentially of two kinds — business driven and data-driven. Business-driven feature engineering revolves around the inclusion of features from a business point of view. The job here is to transform the business variables into features of the problem. In case of data-driven feature engineering, the features you add do not have any significant physical interpretation, but they help the model in the prediction of the target variable.

To apply feature engineering, one must be fully acquainted with the dataset. This involves knowing what the given data is, what it signifies, what the raw features are, etc. You must also have a crystal clear idea of the problem, such as what factors affect the target variable, what the physical interpretation of the variable is, etc.

Question 4) What is the use of regularisation? Explain L1 and L2 regularisations.

Regularisation is a technique that is used to tackle the problem of overfitting of the model. When a very complex model is implemented on the training data, it overfits. At times, the simple model might not be able to generalise the data

and the complex model overfits. To address this problem, regularisation is used.

Regularisation is nothing but adding the coefficient terms (betas) to the cost function so that the terms are penalised and are small in magnitude. This essentially helps in capturing the trends in the data and at the same time prevents overfitting by not letting the model become too complex.

● L1 or LASSO regularisation: Here, the absolute values of the coefficients are added to the cost function. This can be seen in the following equation; the highlighted part corresponds to the L1 or LASSO regularisation. This regularisation technique gives sparse results, which lead to

● **L2 or Ridge regularisation**: Here, the squares of the coefficients are added to the cost function. This can be seen in the following equation, where the highlighted part corresponds to the L2 or Ridge regularisation.

Question 5:

● How to choose the value of the parameter learning rate (α)?

Selecting the value of learning rate is a tricky business. If the value is too small, the gradient descent algorithm takes ages to converge to the optimal solution. On the other hand, if the value of the learning rate is high, the gradient descent will overshoot the optimal solution and most likely never converge to the optimal solution.

To overcome this problem, you can try different values of alpha over a range of values and plot the cost vs the number of iterations. Then, based on the graphs, the value corresponding to the graph showing the rapid decrease can be chosen.

The aforementioned graph is an ideal cost vs number of iterations curve. Note that the cost initially decreases as the number of iterations increases, but after certain iterations, the gradient descent converges and the cost does not decrease anymore.

If you see that the cost is increasing with the number of iterations, your learning rate parameter is high and it needs to be decreased.