1. Sampling Methods

There are different sampling methods which are classified as either probability or non-probability. Probability sampling methods based on random selection. More specifically, each sample from the population of interest has a known probability of selection under a given sampling scheme. In non-probability sampling, members are selected from the population in some nonrandom manner. (StatPac 2012.)

1. Probability sampling methods

Random sampling is the purest form of probability sampling. Each member of the population has an equal and known chance of being selected. The purpose of a random sample - to make sure that the final sample, that is measured, is representative for the entire population from which it was taken. (StatPac 2012.)

Systematic sampling, also known as Nth name selection technique, is often used instead of random sampling. Systematic sampling involves selection of items from the list with a certain step, for example - every 10th item. In this case the reference point can be chosen randomly. (StatPac 2012.)

Stratified sampling is commonly used probability method that is superior to random sampling because it reduces sampling error. To improve the efficiency of sampling it uses information about the subgroups that exist in its basis. If we are using a stratified sampling, efficiency is increased due to the fact that the accuracy of research is growing faster than its cost. (StatPac 2012.)

2. Non-probability sampling methods

Convenience sampling is used in exploratory research where the researcher is interested in getting an inexpensive approximation of the truth. It is the most convenient sampling method. To create a convenient sample, the researcher simply uses volunteers from the people who are corresponding with basic requirements of the study. (StatPac 2012.)

Judgment sampling is a common non-probability method. The researcher selects the sample based on judgment. This is usually an extension of convenience sampling. For example, a researcher may decide to draw the entire sample from one "representative" city, even though the population includes all cities. (StatPac 2012.)

Quota sample is usually applied in the presence of a researcher's prior knowledge on a number of significant (or correlated with them) characteristics of the population. Like stratified sampling, the researcher first identifies the stratums and their proportions as they are represented in the population. (StatPac 2012.)

Snowball sampling is a special non-probability method used when the desired sample characteristic is rare. The method when the respondent recommends you the next respondent. It used in expert surveys on very narrow topics. The method of snowball is often applied research agencies in the recruitment of respondents for the focus groups. In this case, the effectiveness of focus groups strongly decreases. Sometimes a respondent, who is recommending another respondent, may not fully understand the requirements of selection. (StatPac 2012.)