SYSTEMATIC RANDOM SAMPLING

If you have a big, unnumbered sampling frame, like the 51,413 students at the University of Florida in 2008, then simple random sampling is nearly impossible. You would have to number all those names first. Instead, you can do systematic random sampling. For this, you need a random start and a sampling interval, N. You enter the sampling frame at the random start and take every Nth person (or item) in the frame. If you have a printout of 51,413 names, listed 400 to a page, select a single random number between 1 and 51,413. If the random number is 9,857, the listing will be 257 names down from the top of page 25.

The sampling interval depends on the size of the population and the number of units in your sample. If there are 51,413 people in the population, and you are sampling 400 of them, then after you enter the sampling frame (the list of 51,413 names) you need to take every 128th person (400 X 128 = 51,200) to ensure that every person has at least one chance of being chosen. If there are 640 people in a population, and you are sampling 200 of them, then you would take every 4th person. If you get to the end of the list and you are at number 2 in an interval of 4, just go to the top of the list, start at 3, and keep on going (box 5.1).