Basic Statistics and Data Analysis

Statistics Lecture Notes, Online MCQs

Category: Logistic Regression

Logistic Regression

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Binary Logistic Regression Minitab Tutorial

Binary Logistic Regression is used to perform logistic regression on a binary response (dependent) variable (a variable only that has two possible values, such as presence or absence of a particular disease, this kind of variable is known as dichotomous variable i.e binary in nature).

Binary Logistic Regression can classify observations into one of two categories. These classifications can give fewer classification errors than discriminant analysis for some cases.

The default model contains the variables that you enter in Continuous predictors and Categorical Predictors. You can also add interaction and/or polynomial terms by using the tools available in the model sub-dialog box.

Minitab stores the last model that you fit for each response variable. This stored models can be used to quickly generate predictions, contour plots, surface plots, overlaid contour plots, factorial plots, and optimized responses.

To perform a Binary Logistic Regression Analysis in Minitab, follow the steps given below. It is assumed that you have already launched the Minitab software.

Step1:  Choose Stat > Regression > Binary Logistic Regression > Fit Binary Logistic Model.

Step2:  Do one of the following:

If your data is in raw or frequency form, follow these steps:

Response in Binary Logistic Regression (Frequency Format)

The response in Binary Logistic Regression (Frequency Format)

  1. Choose Response in binary response/frequency format, from combobox on top
  2. In Response text box, enter the column that contains the response variable.
  3. In Frequency text box, enter the optional column that contains the count or frequency variable.

If you have summarized data, then follow these steps:

Response in Binary Logistic Regression (Trial Format)

The response in Binary Logistic Regression (Trial Format)

  1. Choose Response in event/trial format, from combo box on top of the dialog box.
  2. In Number of events, enter the column that contains the number of times the event occurred in your sample at each combination of the predictor values.
  3. In Number of trials, enter the column that contains the corresponding number of trials.

Step4:  In Continuous predictors, enter the columns that contain continuous predictors. In Categorical predictors, enter the columns that contain categorical predictors. You can add interactions and other higher order terms to the model.

Step5:  If you like, use one or more of the dialog box options, then click OK.

The following are options available in the main dialog box of Minitab Binary Logistic Regression:

The response in binary response/frequency format: Choose if the response data has been entered as a column that contains 2 distinct values i.e as a dichotomous variable.
Response: Enter the column that contains the response values.
Response event: Choose which event of interest the results of the analysis will describe.
Frequency (optional): If the data are in two columns i.e. one column that contains the response values and the other column that contains their frequencies then enter the column that contains the frequencies.
Response in event/trial format: Choose if the response data are two columns – one column that contains the number of successes or events of interest and one column that contains the number of trials.
Event name: Enter a name for the event in the data.
Number of events: Enter the column that contains the number of events.
Number of trials: Enter the column that contains the number of nonevents.
Continuous predictors: Select the continuous variables that explain changes in the response. The predictor is also called the X variable.
Categorical predictors: Select the categorical classifications or group assignments, such as the type of raw material, that explain changes in the response. The predictor is also called the X variable.

Step 6: To stores diagnostic measures and characteristics of the estimated equation click Storage… button.

Binary Logistic Regression Storage Dialog Box

Binary Logistic Regression Storage Dialog Box

Logistic Regression

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Logistic regression Introduction

Logistic regression was introduced in the 1930s by Ronald Fisher and Frank Yates and was first proposed in 1970s as an alternative technique to overcome 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, 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. 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 so 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.

References:

Basic Statistics / Logistic Regression / Probability

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Introduction Odds Ratio

Introduction Odds Ratio

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.

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. When computing Odds Ratio, one would do:

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

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

The Odds Ratio is the ratio of two odds can be used to quantify how much a factor is associated to the response factor in a given model. If the probabilities of occurrences an event are $P_1$ (for first group) and $P_2$ (for 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 ith 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 associated with a one unit increase in the independent variable.