Simple random sampling is a simple and widely used method of sampling in which the samples with equal probability of being selected at each draw, are selected unit by unit. It is a technique of drawing a sample in such a way that each population unit has an equal and independent chance of being included in the sample. There are two ways of drawing a sample through simple random sampling:

- Simple random sampling with replacement: In this sampling, an item previously drawn is replaced before the next draw in which the same unit may be included more than once in the sample.
- Simple random sampling without replacement: In this method, an item previously drawn is not replaced before the next draw in which all units in the sample are distinct.

**Simple random sampling with replacement (SRSWR):**A simple random sampling with replacement (SRSWR) of size*n*from a population of*N*units can be imagined as an act of drawing independent samples of size 1. A unit is randomly selected from the population to be the first sampled unit, with probability 1/*N*. Then, the sampled unit is replaced in the population, and a second unit is randomly selected with probability 1/*N*. This procedure is repeated until the sample has*n*units, which may include duplicates from the population. The number of possible sample having sample size*n*from the population size*N*is*N*. Hence, the probability of one sample = 1/^{n}*N*.^{n}

However, in finite population sampling, sampling the same person twice provides no additional information. We usually prefer to sample without replacement, so that the sample contains no duplicates.**Simple random sampling without replacement (SRSWOR):**A simple random sample without replacement (SRSWOR) of size*n*is selected so that every possible subset of*n*distinct units in the population has the same probability of being selected as the sample. One unit is randomly selected from the population to be first sampled unit with probability 1/*N*. The sampled unit is not replaced and the second unit is randomly selected with probability 1/(*N*-1). Similarly, the second sampled unit is not replaced and third unit is randomly selected with probability 1/(*N*-2) and so on until the sample has*n*units obtained from the population. There arepossible samples of size^{N}C_{n}*n*from population size*N*and each is equally likely. So, the probability of selecting any individual sample of*n*units is 1/.^{N}C_{n}