Statistics for the Behavioral Sciences by Frederick J. Gravetter, Larry B. Wallnau ISBN 10: 1305504917 ISBN 13: 9781305504912

PREVIEW
In a controversial study, Bem (2011) appears to show that
participants are able to anticipate and benefit from randomly
selected events that will occur in the future. That’s
right. The individuals in the study were able to improve
their performance by using information that was not made
available until after their performance was tested. One of
the many experiments reported was a demonstration of how
practice and rehearsal can improve memory performance,
which is not surprising except that the practice occurred
after the memory test. In the study, participants were
shown a list of 48 words (common nouns), one at a time
for 3 seconds each. The words corresponded to 12 foods,
12 animals, 12 occupations, and 12 items of clothing. As
each word was presented, the participants were asked to
form a mental image of the object represented by the word.
At the end of the list, they were given a surprise memory
test and asked to type as many words as they could
remember in any order. After the memory test, 24 of the
words were randomly selected with the restriction that the
24 words consisted of 6 words from each of the 4 categories
(6 foods, 6 animals, and so on). The participants then
practiced these 24 words using the following procedure:
1. The complete list of 24 words was shown on a
computer screen in a randomized order. The participant
was asked to identify the 6 food words
by clicking on them, which highlighted them.
The participant then typed the 6 selected words
into 6 empty boxes on the screen.
2. This process was then repeated for the other
3 categories until each participant had identified
and typed all of the 24 words.
The researchers then returned to the results from the
original memory test for each participant and computed a
memory score for the 24 words that had been practiced and
a separate memory score for the 24 no-practice words. The
results showed that memory for the practiced words was
significantly better than memory for the no-practice words,
even though the practice occurred after the memory test.
Is this a scientific demonstration of psychic power?
You may be somewhat skeptical, especially when you
learn that the participants were regular Cornell undergraduate
students who were not selected because they
had unique ESP abilities. Because this backward causeand-effect
relationship is completely counterintuitive,
many behavioral scientists were immediately skeptical.
Several groups of researchers attempted to replicate
Bem’s results but repeatedly failed (Yong, 2012).
If Bem’s study is not an example of ESP, then is there
another possible explanation for the result? The answer is
“yes” and the result might be explained by variability. Variability
is an everyday concept as well as a statistic concept.
In both cases, it simply means that things aren’t always the
same; they vary. If you are selecting individuals from the
class list for an Introductory Psychology course, you will
get a different person on each selection. They will have
different ages, heights, personalities, IQs, and so on. In the
same way, if you conduct a research study over and over,
you will get different results every time. Usually, however,
the results will be similar. The students you select will usually
be 18- or 19-years-old, above average intelligence, and
fairly typical college freshmen and sophomores. The results
you obtain from repeating a research study will also tend to
be similar from one time to the next. In some cases, however,
an extreme outcome occurs. Just as it is possible to toss
a coin and get 10 heads in a row, you may select a student
who happens to be a 48-year-old lawyer who is going back
to school to start a new career as a social worker. It also is
possible to obtain really extreme results for a research study.
Fortunately, these extreme outcomes are very rare
and researchers use this fact as a demonstration that their
research studies are very unlikely to be extreme outcomes
that occurred just by chance. If you give a new cholesterol
medication to 25 patients and all 25 show a drop in cholesterol,
then you can be reasonably confident that the medication
works. If only 14 out of 25 had lower levels, then
the results could be explained as just chance, but 25 out
of 25 is very unlikely to have occurred by chance alone.
For Bem’s experiment the obtained outcome was found to
be “very unlikely” to have occurred by chance. Therefore,
chance is eliminated as a plausible explanation and we are
left with the conclusion that there is some force at work
that causes better performance for the practice words.
The problem comes from the fact that “very unlikely”
does not mean the same thing as “impossible.” Because
outcomes are variable, you will occasionally obtain an
extreme, very unlikely outcome simply by chance. Perhaps
it is just chance, rather than ESP, that explains Bem’s results.
In this chapter we introduce the statistical concept of
variability. We will describe the methods that are used to
measure and objectively describe the differences that exist
from one score to another within a distribution. In addition
to describing distributions of scores, variability also
helps us determine which outcomes are likely and which
are very unlikely to be obtained. This aspect of variability
will play an important role in inferential statistics.
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