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To use a computer program (called a random number generator) you must make sure that you give each of the people in your population a number. The program gives you a list of randomly selected numbers within the range you provide. After getting the random numbers, identify the people with those selected numbers and try to get them to participate in your research study!

If you decide to use a table of random numbers (such as the one in Figure 9.1 in you book), here’s what you need to do. First, pick a place to start, and then move in one direction (e.g., move down the columns). Use the number of digits in the table that is appropriate for your population size (e.g., if there are 2500 people in the population then use 4 digits). Once you get the number of selected numbers needed, find out who those people are and get them to participate in your research study. Note: if you get the same number twice, just ignore it and move on to the next number.

Systematic Sampling

Systematic sampling is the second type of random sampling.

It is an equal probability sampling method (EPSEM).

Remember simple random sampling was also an EPSEM.

Systematic sampling involves these three steps:

First, determine the sampling interval, which is symbolized by "k," (it is the population size divided by the desired sample size).

Second, randomly select a number between 1 and k, and include that person in your sample.

Third, include each kth element in your sample. For example if k is 10 and your randomly selected number between 1 and 10 was 5, then you will select persons 5, 15, 25, 35, 45, etc.

When you get to the end of your sampling frame you will have all the people to be included in your sample.

One potential (but rarely occurring) problem in systematic sampling is called periodicity (i.e., there is a cyclical pattern in the sampling frame). It can occur when you attach several ordered lists to one another (e.g., if you take lists from multiple teachers who have all ordered their individual class lists according to a variable such as grades or IQ). Stratification like this is not a problem if you have only one list. Basically, if you ever attach multiple lists to one another, there could be a problem. It is better to reorganize your lists into one overall list (i.e., into a new, unordered sampling frame).

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