Chapter 9 talks about the importance of a factorial design and its efficiency in testing two or more factors in an experiment. It can also be used to measure main effects and interactions, which make it a major element in psychology by providing valuable information that other experiments cannot. Factorial designs can be described or assessed by shorthand notations and statistics. If their are 4 numbers that means there is 4 factors and the levels of these factors are based on the value of these numbers. Stats are then used to determine the effect in a design. There are some limitations however, such as the requisite for many subjects. The experiments themselves may be extremely long and require arduous statistical procedures. Chapter 10 talks about within-subject designs that allow each subject to be part of a number of conditions in an experiment. The main advantage to this is that it allows us to compare the behavior of the same subjects under different treatment conditions. It does have its problems though such as control problems, in which we have to control a couple of different variables, such as order effects and carryover effect. Order effects are the positive and negative changes in performance that occur in a different number of treatments. We can control this in a different number of ways such as counterbalancing in which we distribute the progressive errors across the different treatments in the experiment. Another way is by subject-by-subject in which we control the error for each individual rather than the entire treatment. Adding all the individual error data for each condition can also use across-subjects counterbalancing. The second type of problem is the carryover effect, in which the treatment conditions affect one another. Counterbalancing as well can control this problem.