# Resampling

## What is Resampling :

Resampling is a statistical method used to analyze and understand a dataset by generating new samples from it. It allows for the estimation of statistical properties, such as mean and variance, and can be used to test hypotheses, assess model performance, and select the best models for a given dataset.
There are two main types of resampling: bootstrapping and cross-validation.
Bootstrapping is a resampling method that involves randomly sampling with replacement from a dataset to create a new sample. This process can be repeated multiple times, resulting in a collection of samples known as a bootstrap sample. The mean and standard deviation of the bootstrap samples can be used to estimate the mean and standard deviation of the original dataset.
For example, consider a dataset of 10 individuals with their respective heights (in inches). The mean height is 67 inches and the standard deviation is 4 inches. To generate a bootstrap sample, we could randomly select heights from the original dataset with replacement and calculate the mean and standard deviation of the new sample. If we repeat this process 100 times, we will have 100 bootstrap samples with corresponding means and standard deviations. By examining the distribution of the means and standard deviations, we can get a better understanding of the spread and variability of the original dataset.
Cross-validation is a resampling method used to evaluate the performance of a machine learning model. It involves dividing the dataset into k equal-sized folds and training the model on k-1 folds while evaluating it on the remaining fold. This process is repeated k times, with a different fold serving as the evaluation set each time. The performance metrics, such as accuracy and F1 score, are then averaged across all k iterations to provide an estimate of the model’s generalization performance.
For example, suppose we have a dataset of 1000 observations and we want to build a classification model to predict whether an individual will have heart disease. We can use 10-fold cross-validation to evaluate the performance of our model. The dataset is divided into 10 equal-sized folds, with 100 observations in each fold. We train the model on the first 9 folds and evaluate it on the 10th fold. We then repeat this process, with each of the 10 folds serving as the evaluation set once. We calculate the accuracy of the model on each iteration and average the results to get an overall estimate of the model’s performance. This allows us to assess the model’s ability to generalize to new data, rather than just its performance on the training data.
Both bootstrapping and cross-validation have their strengths and limitations. Bootstrapping is useful for estimating statistical properties of a dataset, but it relies on the assumption that the original sample is representative of the population. Cross-validation is useful for evaluating the performance of machine learning models, but it may not be as effective for small datasets where there are not enough observations to split into multiple folds.
Overall, resampling is a powerful tool for understanding and analyzing datasets and evaluating machine learning models. It allows for the estimation of statistical properties and the assessment of model performance, which are essential tasks in data science and machine learning.