People remember concrete words (e.g., dog, elephant, or chair) better than abstract ones (love, freedom, or desire).

This conclusion likely came from an experiment.  Such an experiment could have been set up in two different ways. In one, called a between-subjects design, two groups of people are created by randomly assigning people to one of two groups.  One group learns concrete words; the other learns abstract ones.  Researchers compare the memory of the two groups to determine which group remembered better.  In another experimental design, called a within-subjects design, all participants learn both the concrete and abstract words (probably randomly mixed together on one list).  Researchers examine whether individuals in general learn concrete words better than abstract ones.