You would not want to make conclusions about memory by examining only one person. Therefore, you should use more than one subject in this experiment.
Let's assume that you will use 16 subjects in your experiment. You will need to make some decisions about how to measure short-term recall in your 16 subjects.
Recall the original question: "Is it easier to remember strings of characters that spell familiar words than to remember arbitrary strings of characters?"
As stated, the question is perhaps not as specific as it should be. For example, we have not clarified the population of people we are studying. Age may have an impact on a person's ability to memorize. Are we interested in adults only?
If you believe that age makes a difference in a person's recall ability, then perhaps you need to refine the question.
Below are five different ways that you might assign the 16 subjects to groups and collect your measurements. These are referred to as designs. You should consider only one design at a time; do not move on to Design 2 until you have answered all four questions for Design 1, and so on. Your focus should be on the methods of data collection for each design.