It is a common practice to pick one group from a population to study in order to better understand the entire population.

Consider the case where we would like to know the proportion of iPhone users in a particular city. Calling everyone in the city and asking what kind of phone they use is one way to go about this. The alternative would be to ask the same question of a smaller subgroup of people and then use the results to estimate the size of the entire population.

But this procedure is more complicated than it seems. Your sample size needs to be perfect whenever you use this method; it shouldn't be either too big or too small. Once the size of your sample has been determined, you must then collect a sample from the population using the appropriate sampling techniques. Every sampling method ultimately falls into one of two broad categories:

Probability sampling: The sample is chosen using random selection methods.

Non-probability sampling: To choose the sample, non-random selection methods based on particular criteria are used.