Simple Random Sample

One method of sampling is a simple random sample. Using this design we select n units from the population in such a way that each sample has the same chance of being selected. In other words, every experimental unit has the same probability of being selected to be surveyed.

Example

If we wanted to collect information about public opinion of the Forest Service in a particular town we could simply get a list of all the residents of that town, and randomly choose names from the list to take our survey.

Stratified Random Sample

Another approach, which is similar to simple random sampling is stratified random sampling. Using this approach we divide the population into two or more groups based on some factor, such as age or income. Once the population is stratified we then use the principals of a simple random sampling design in each stratification.

In the example above, we may be concerned about the opinion of men and women about the forest service in this town. So we would randomly choose names from a list of all the men and all the women who live in the town to take our survey.

Systematic Sampling

Systematic sampling is another technique often used when conducting a survey. Under this design we may sample every tenth person from the population. Often time’s systematic sampling will be called random systematic sampling. This implies that a random starting point was used and then every tenth person was sampled.

Example