The within-subject design can be used in a circumstance where a subject can be useful in more than one condition. In chapter 11, it mentions an experiment that involved the timeframe in which infants of a certain age begin to smile. Since no child’s psyche is shaped the exact same way, they will have different degrees of which they smile, cry, or giggle for example. This is where the within-subject is useful because the experiment can be tailored for just the infant and mother, a large subject pool is not needed. (Myers & Hansen, 2012).
The between-subject design involves multiple subjects in each treatment condition. The number of subjects really depends on what your goal is for the experiment. If there is chance that the subjects will be different it is good to go with a large number and if the subjects are similar, it is good to go with a smaller number. The environment where you are recruiting subjects can give you an idea if the subjects will be like each other or if they will be quite different (Myers & Hansen, 2012. A circumstance that might call for a between subject design is if you have access to a large pool of subjects to extract data from.
A small N design can be used on one person, which is incredibly convenient. The experiment can be used in short time spans (days or weeks) or long-time spans (months or years), it is thought that small N designs are more precise, and its proponents believe that large subject pools can wreck validity of the results (Myers & Hansen, 2012). A circumstance where you might see a small N design experiment is during the testing of a new psychotherapy that a team of researchers designed.
A large N design is a between subject design. This design uses combined data from multiple subjects so the researchers can reach their conclusions after the independent variable is applied (Myers & Hansen, 2012). If there is a group of 200 people, this is a circumstance that will likely call for a large N design. It would take far too long to conduct the experiment if you took the small N approach to this group of subjects.