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Experimental Design in Psychological Research 117

Figure 6.1
In a controlled experiment,subjects are randomly assigned to conditions, and differences between
groups are measured.

A controlled experiment in medical research might seek to discover if a cer-
tain food additive causes cancer. The researcher might randomly divide a group
of laboratory mice into two smaller groups,giving the food additive to one
group and not to the other. The variable he/she is interested in is the effect of
the food additive; in the language of experimental design,this is called the
‘‘independent variable.’’ After a period of time,the researcher compares the
mortality rates of the two groups; this quantity is called the ‘‘dependent vari-
able’’ (figure 6.1). Suppose the group that received the additive tended to die
earlier. In order to deduce that the additive caused the difference between the
groups,the conditions must have been identical in every other respect. Both
groups should have had the same diet,same feeding schedule,same tempera-
ture in their cages,and so on. Furthermore,the two groups of mice should have
started out with similar characteristics,such as age,sex,and so on,so that these
variables—being equally distributed between the two groups—can be ruled
out as possible causes of the difference in mortality rates.
The two key components of a controlled experiment are random assignment of
subjects,and identical experimental conditions (see figure 6.1). A researcher might
have a hypothesis that people who study for an exam while listening to music
will score better than people who study in silence. In the language of experi-
mental design,music-listening is the independent variable,and test performance,
thequantitytobemeasured,is the dependent variable.
No one would take this study seriously if the subjects were divided into two
groups based on how they did on the previous exam—if,for instance, the
top half of the students were placed in the music-listening condition,and the