Sampling schemes may be:
- without replacement ('WOR'—no element can be selected more than once in the same sample)
- or with replacement ('WR'—an element may appear multiple times in the one sample).
If we catch fish, measure them, and immediately return them to the water before continuing with the sample, this is a WR design, because we might end up catching and measuring the same fish more than once. However, if we do not return the fish to the water (e.g., if we eat the fish), this becomes a WOR design.