## Logistic regression Introduction (2015)

Logistic regression was introduced in the 1930s by Ronald Fisher and Frank Yates and was first proposed in the 1970s as an alternative technique to overcome the limitations of ordinary least square regression in handling dichotomous outcomes. It is a type of probabilistic statistical classification model which is a non-linear regression model, and can be converted into a linear model by using a simple transformation. It is used to predict a binary response categorical dependent variable, based on one or more predictor variables. That is, it is used in estimating empirical values of the parameters in a model. Here response variable assumes a value as zero or one i.e., dichotomous variable.

#### Logistic Regression Model

It is the regression model of $b$, $a$ logistic regression model is written as

$\pi=\frac{1}{1+e^{-[\alpha +\sum_{i=1}^k \beta_i X_{ij}]}}$

where $\alpha$ and $\beta_i$ are the intercept and slope respectively.

So in simple words, logistic regression is used to find the probability of the occurrence of the outcome of interest.  For example, if we want to find the significance of the different predictors (gender, sleeping hours, took part in extracurricular activities, etc.), on a binary response (pass or fail in exams coded as 0 and 1), for this kind of problems we used logistic regression.

By using a transformation this nonlinear regression model can be easily converted into a linear model. As $\pi$ is the probability of the events in which we are interested if we take the ratio of the probability of success and failure then the model becomes a linear model.

$ln(y)=ln(\frac{\pi}{1-\pi})$

The natural log of odds can convert the logistics regression model into a linear form.

Binary Logistic Regression in Minitab

References:

## Introduction Odds Ratio (2015)

### Introduction Odds Ratio

An odds ratio is a relative measure of effect, allowing the comparison of the intervention group of a study relative to the comparison or placebo group.

Medical students, students from clinical and psychological sciences, professionals allied to medicine enhancing their understanding and learning of medical literature, and researchers from different fields of life usually encounter Odds Ratio (OR) throughout their careers.

When computing the OR, one would do:

• The numerator is the odds in the intervention arm
• The denominator is the odds in the control or placebo arm= OR

### Calculating ODDs Ratios

The ratio of the probability of success and failure is known as the odds. If the probability of an event is $P_1$ then the odds are:
$OR=\frac{p_1}{1-p_1}$

If the outcome is the same in both groups, the ratio will be 1, implying that there is no difference between the two arms of the study. However, if the $OR>1$, the control group is better than the intervention group while, if the $OR<1$, the intervention group is better than the control group.

The Odds Ratio is the ratio of two odds that can be used to quantify how much a factor is associated with the response factor in a given model. If the probabilities of occurrences of an event are $P_1$ (for the first group) and $P_2$ (for the second group), then the OR is:
$OR=\frac{\frac{p_1}{1-p_1}}{\frac{p_2}{1-p_2}}$

If predictors are binary then the OR for $i$th factor is defined as
$OR_i=e^{\beta}_i$

The regression coefficient $b_1$ from logistic regression is the estimated increase in the log odds of the dependent variable per unit increase in the value of the independent variable. In other words, the exponential function of the regression coefficients $(e^{b_1})$ in the OR is associated with a one-unit increase in the independent variable.