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Experimental Design in Psychological Research 125

complete the experiment with each order. In a between-subjects design,different
subjects would be assigned different orders. The choice will often depend on
the available resources (time and availability of subjects). The number of pos-
sible orders is N! (‘‘n factorial’’),where N equals the number of stimuli. With
two stimuli there are two possible orders ð2! ¼ 2 1Þ; with three stimuli there
are six possible orders ð3! ¼ 3 2 1Þ; with six stimuli there are 720 possible
orders ð6! ¼ 6 5 4 3 2 1Þ. Seven hundred twenty orders is not practical
for a within-subjects design,or for a between-subjects design. One solution in
this case is to create an order that presents each stimulus in each serial position.
A method for accomplishing this involves using the Latin Square. For even-
numbered N,the size of the Latin Square will be N N; therefore,with six
stimuli you would need only 36 orders,not 720. For odd-numbered N,the size
of the Latin Square will be N 2N. Details of this technique are covered in ex-
perimental design texts such as Kirk (1982) and Shaughnessy and Zechmeister
(1994).

6.7 Ethical Considerations in Using Human Subjects
Some experiments on human subjects in the 1960s and 1970s raised questions
about how human subjects are treated in behavioral experiments. As a result,
guidelines for human experimentation were established. The American Psy-
chological Association,a voluntary organization of psychologists,formulated a
code of ethical principles (American Psychological Association 1992). In addi-
tion,most universities have established committees to review and approve re-
search using human subjects. The purpose of these committees is to ensure that
subjects are treated ethically,and that fair and humane procedures are fol-
lowed. In some universities,experiments performed for course work or experi-
ments done as ‘‘pilot studies’’ do not require approval,but these rules vary
from place to place,so it is important to determine the requirements at your
institution before engaging in any human subject research.
It is also important to understand the following four basic principles of ethics
in human subject research:

1. Informed consent. Before agreeing to participate in an experiment,sub-
jects should be given an accurate description of their task in the experi-
ment,and told any risks involved. Subjects should be allowed to decline, or
to discontinue participation in the experiment at any time without penalty.
2. Debriefing. Following the experiment,the subjects should be given an
explanation of the hypothesis being tested and the methods used. The ex-
perimenter should answer any questions the subjects have about the pro-
cedure or hypothesis. Many psychoacoustic experiments involve difficult
tasks,leading some subjects to feel frustrated or embarrassed. Subjects
should never leave an experiment feeling slow,stupid,or untalented. It is
the experimenter’s responsibility to ensure that the subjects understand
that these tasks are inherently difficult,and when appropriate,the sub-
jects should be told that the data are not being used to evaluate them
personally,but to collect information on how the population in general
can perform the task.