In an experiment, although subjects may not be selected randomly, they should be randomly assigned to either the experimental or control condition. The purpose of random assignment is to make sure that, except for differences, groups of subjects are similar with respect to any . It is usually desirable that numbers of subjects be assigned to the experimental and control groups. To accomplish this, assignment should be done in Section II refers to the proportion or fraction of times that a particular outcome will occur. Eventually, we will want to specify the probability of a particular outcome. Probability can be determined by or . Probability values can vary between and , and a set of probability values will always sum to