Bootstrapping is a uniform sampling with replacement from a given training set, that is, whenever an example is selected, it is equally likely to be selected again and added to the training set again.

The bootstrap method was published by Bradley Efron in Annals of Statistics in 1979. When the sample comes from a population that can be described by a normal distribution, its sampling distribution is a normal distribution. However, when the population from which the sample comes cannot be described by a normal distribution, it is analyzed using asymptotic analysis, bootstrap methods, etc.