In *probability sampling*, the sample is selected in such a way that each unit within the population or universe has a known chance of being selected. It is this concept of "known chance" that allows for the statistical projection of characteristics based on the *sample* to the *population*.

Most estimates tend to cluster around the true population or universe mean. When plotted on a graph, these means form what is called the *normal or bell curve.* This theoretical distribution allows for the calculation of the probability of a certain event occurring (e.g. the likelihood that an activity studied will be undertaken by people over 65 years old, if those are the variables being studied).

There are three main types of probability or random sampling that we will review more closely: