Understanding Psychology (10th Ed)

Using Statistics to Answer 
Questions: Inferential Statistics 
and Correlation
The Correlation Coeffi
cient: Measuring 
Relationships
How can we assess the variability of a set of 
data? 
Mean=50
b.
Mean
Median
Mode
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Prologue
Selma Vorwerk
As the boat moved nearer to shore, the outline of the Statue of 
Liberty was plainly visible in the distance. Closer and closer it 
came, sending a chill down the Selma Vorwerk’s spine. A symbol 
of America, the statue represented the hopes she carried from 
her native Europe in the early 1900s—hopes of liberty, of success, 
of a life free of economic and social strain. 
Yet as the boat sailed closer to Ellis Island, the fi rst point of 
arrival in the United States, Vorwerk did not realize that her very 
presence—and that of the other thousands of immigrants 
seeking their fortune in a land of opportunity—was threatened. 
A strong political movement was growing in the country on 
which she was pinning her hopes. That movement sought, by 
using information collected by psychologists, to stem the fl ow 
of immigrants through “scientifi c” analysis of data. 
The groups’ major claim was that a fl ood of “mentally defi -
cient” immigrants was poisoning the intellectual capacity of the 
United States. To supporters of that view, unless drastic measures 
were taken, it would not be too many years before Western 
civilization collapsed from a lack of intelligence. 
To support this assertion, Lathrop Stoddard, a member of 
the anti-immigration movement, reported the results of a study 
of intelligence in which tests were administered to a group of 
82 children and 400 adults. On the basis of those test results, he 
concluded that the average mental age of people in the United 
States was only 14 years—proof to him that unlimited immigra-
tion had already produced a serious decline in the country’s 
intelligence. 

Looking
Ahead 

Fortunately for immigrants such as Selma Vorwerk, observers in 
favor of immigration pointed out the fallacy of using data from 
a relatively small sample—when a considerably larger set of 
intelligence test data was available. The U.S. Army had been 
collecting intelligence data for years and had the test scores of 
1.7 million men available. When those scores were analyzed, it 
immediately became apparent that the claim that the average 
mental age of American adults was 14 years was completely 
without merit. 
A debate reminiscent of this one rages today as some 
observers suggest that an unrestrained fl ow of immigrants—this 
time from Latin America and Asia—will seriously damage the 
United States. This time, though, the debate is based more on 
analyses of social and economic statistics: opponents of immigra-
tion suggest that the social fabric of the country will be changed 
and that jobs are being taken away from longer-term residents 
because of the infl ux of immigrants. Equally vehement propo-
nents of immigration suggest that the relevant statistics are 
being misinterpreted and that their analyses of the situation 
result in a very diff erent conclusion.
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