The most common type of sampling design is the simple random
sampling. In fact, most statistical equations assume that you have
conducted a simple random sample. Using this approach every
possible combination of experimental units has an equal and
independent chance of being selected. This approach implies that two
conditions must be met first that every experimental unit has an
equal chance of being selected and second that the outcome of one
selection is not dependent upon the outcome of another selection.
You could simply do this by assigning every unit a number and then
randomly pick x numbers for sampling.
Figure 1. A possible
arrangement for 56 samples using a simple random sampling design |