Russel-Research-Method-in-Anthropology

Sampling 151
they’re joined by all the single people home from work, so the average family
size goes up.
Telephone survey researchers call back from three to 10 times before
replacing a member of a sample. When survey researchers suspect (from prior
work) that, say, 25% of a sample won’t be reachable, even after call-backs,
they increase their original sample size by 25% so the final sample will be
both the right size and representative.
Systematic Random Sampling
Most people don’t actually do simple random sampling these days;
instead they do something called systematic random sampling because it is
much, much easier to do. If you are dealing with an unnumbered sampling
frame of 48,673 (the student population at the University of Florida in 2003),
then simple random sampling is nearly impossible. You would have to number
all those names first. In doing systematic random sampling, you need a random
start and a sampling interval, N. You enter the sampling frame at a randomly
selected spot (using appendix A again) and take every Nth person (or
item) in the frame.
In choosing a random start, you only need to find one random number in
your sampling frame. This is usually easy to do. If you are dealing with 48,673
names, listed on a computer printout, at 400 to a page, then number 9,457 is
257 names down from the top of page 24.
The sampling interval depends on the size of the population and the number
of units in your sample. If there are 10,000 people in the population, and you
are sampling 400 of them, then after you enter the sampling frame (the list of
10,000 names) you need to take every 25th person (400 25 10,000) 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.
Periodicity and Systematic Sampling
I said that systematic sampling usually produces a representative sample.
When you do systematic random sampling, be aware of the periodicity problem.
Suppose you’re studying a big retirement community in South Florida.
The development has 30 identical buildings. Each has six floors, with 10
apartments on each floor, for a total of 1,800 apartments. Now suppose that
each floor has one big corner apartment that costs more than the others and