A random (probability) sample is a sample, where each element of the population has a certain, predetermined probability of being selected. This allows a researcher to calculate how well the sample represents the population, from which it is allocated (designed). This selection is sometimes also called random sampling.
Probabilistic sampling methods include:
♦ simple random sampling,
♦ systematic sampling,
♦ cluster sampling,
♦ stratified sampling.
Simple random sampling is a kind of probabilistic sampling method. Data selection is based on the assumption that the probability of being selected in the sample is known and is the same for all the cases together. The probability of being included in the sample is determined by the ratio of the sample size to the size of the population. For example, a restaurant manager wants to determine the proportion of bills with a total price of $50 and above in the whole number of bills paid this month. In this case, the population is all prices of bills that were paid during this month. A manager may randomly select a number of bills, let’s say, 100 bills. This will be a simple random sample of 100 bills.
Stratified sampling is more accurate than simple random sampling. This method is especially good when the general population is heterogeneous. In this case, simple random sampling is extremely inefficient. However, stratified sampling may be applied only if there is enough information about the population (for example, we need to determine the percentage of men and women, if we want to stratify the sample by gender). The lack of such information makes it impossible to use a stratified sample. For a restaurant manager, it may be useful to know, if menu preferences and age of the clients are related factors. It is possible to divide age on a few groups and divide the orders by age group. As a result, a manager obtains a sample of orders that are classified by age groups. This sample is very convenient to determine the most frequent items ordered. Hence, manager will know preferences of various age groups.
