To accomplish this goal, an experimental study contains the
following four basic elements, which are also shown in Figure 7.1:
1. Manipulation. The researcher manipulates one variable by changing its
value to create a set of two or more treatment conditions.
2. Measurement. A second variable is measured for a group of participants
to obtain a set of scores in each treatment condition.
3. Comparison. The scores in one treatment condition are compared
with the scores in another treatment condition. Consistent differences
between treatments are evidence that the manipulation has caused
changes in the scores (See Box 7.1).
4. Control. All other variables are controlled to be sure that they do not
influence the two variables being examined.