Logistic Regression: When to use ? When do we call the data Linearly separable?

When to use Logistic Regression?

When data is linearly separable and the outcome is binary or dichotomous in nature.

How to identify if Data is Linearly separable or Not  ?

Method 1: Apply  convex hull algorithm to the data to find out whether they are overlapping or not. If they overlap , then data is not linearly separable.

                     

Image source: Google image

Method 2: Using Support Vector Machine with Kernels: The algorithm tries to divide the data using hyper planes. Look out the residuals. If present, then data is not linearly separable.

Image source: Google image

Method 3: Data Visualization : Use intuition to find whether it is separable with straight line or not. Example

              

Image Source:- Test for linear separability