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same number of students from each of those colleges. This ensures that colleges
of different sizes are represented in the sample. You then weight the data from
each college in accordance with the percentage contribution each college makes
to the total student population of your sample. (For further reading,see
Shaughnessy and Zechmeister 1994.)
Choosing subjects randomly requires careful planning. If you try to take a
random sample of Stanford students by standing in front of the Braun Music
Building and stopping every third person coming out,you might be selecting a
greater percentage of music students than actually exists on campus. Yet truly
random samples are not always practical. Much psychological research is con-
ducted on college students who are taking an introductory psychology class,
and are required to participate in an experiment for course credit. It is not at all
clear whether American college students taking introductory psychology are
representative of students in general,or of people in the world in general,so
one should be careful not to overgeneralize findings from these studies.

6.3.2 Correlational Studies
Asecondtypeofstudy is the correlational study (figure 6.2). Because it is not
always practical or ethical to perform random assignments,scientists are
sometimes forced to rely on patterns of co-occurrence,or correlations between
events. The classic example of a correlational study is the link between cigarette
smoking and cancer. Few educated people today doubt that smokers are more
likely to die of lung cancer than are nonsmokers. However,in the history of
scientific research there has never been a controlled experiment with human
subjects on this topic. Such an experiment would take a group of healthy non-
smokers,and randomly assign them to two groups, a smoking group and a
nonsmoking group. Then the experimenter would simply wait until most of the
people in the study have died,and compare the average ages and causes of
death of the two groups. Because our hypothesis is that smoking causes cancer,
it would clearly be unethical to ask people to smoke who otherwise would not.
The scientific evidence we have that smoking causes cancer is correlational.
That is,when we look at smokers as a group,a higher percentage of them do
indeed develop fatal cancers,and die earlier, than do nonsmokers. But without
a controlled study,the possibility exists that there is a third factor—a mysteri-
ous ‘‘factor x’’—that both causes people to smoke and to develop cancer. Per-
haps thereis some enzymeinthe body that givespeopleanicotine craving,
and this same enzyme causes fatal cancers. This would account for both out-
comes,the kinds of people who smoke and the rate of cancers among them,
and it would show that there is no causal link between smoking and cancer.
In correlational studies,a great deal of effort is devoted to trying to uncover
differences between the two groups studied in order to identify any causal fac-
tors that might exist. In the case of smoking,none have been discovered so far,
but the failure to discover a third causal factor does not prove that one does not
exist. It is an axiom in the philosophy of science that one can prove only the
presence of something; one can’t prove the absence of something—it could al-
ways be just around the corner,waiting to be discovered in the next experiment
(Hempel 1966). In the real world,behaviors and diseases are usually brought