Thought and Knowledge: An Introduction to Critical Thinking, 4th Edition (Thought & Knowledge: An Introduction to Critical Thinking) by Halpern Diane F. & Halpern Diane F

Author:Halpern, Diane F. & Halpern, Diane F. [Halpern, Diane F.]
Language: eng
Format: mobi
Tags: Behavioral Sciences
Publisher: Taylor and Francis
Published: 2002-12-17T16:00:00+00:00

Sample Size

Given a thimbleful of facts, we rush to make generalizations as large as a tub.

—Gordon Allport (1954, p. 8)

The number of subjects you include in your sample is called the sample size. Suppose that treatment Program 1 has had six patient/subjects and Program 2 has had 10 patient/subjects. Both of these numbers are too small to determine the success rate of the treatments. (A subject is a person or animal or entity who participates in an experiment.) When scientists conduct experiments, they often use large numbers of subjects because the larger the sample size, the more confident they can be in generalizing the findings to the population. This principle is called the law of large numbers, and it is an important statistical law. If, for some reason, they cannot use a large number of subjects, they may need to be more cautious or conservative in the conclusions that they derive from their research. Although a discussion of the number of subjects needed in an experiment is beyond the scope of this book, it is important to keep in mind that for most everyday purposes, we cannot generalize about a population by observing how only a few people respond. Suppose this happened to you:

After months of deliberation over a new car purchase, you finally decided to buy a Chevrolet Impala. You found that both Consumer Reports and Road and Track magazines gave the Impala a good rating. It is priced within your budget, and you like its circular rear lights and "sharp" appearance. On your way out the door to close the deal, you run into a close friend and tell her about your intended purchase. "An Impala!" she shrieks. "My brother-in-law bought one and if s a tin can. It's constantly breaking down on the freeway. He's had it towed so often that the rear tires need replacing."

What do you do?

Most people would have a difficult time completing the purchase because they are insufficiently sensitive to sample size issues. The national magazines presumably tested many cars before they determined their rating. Your friend's brother-in-law is a single subject. You should place greater confidence in results obtained with large samples than in results obtained with small samples (assuming that the "experiments" were equally good). Yet, many people find the testimonial of a single person, especially if it's someone they know, more persuasive than information gathered from a large sample, especially when there is preference for the results obtained from the small sample (Klacznski, Gordon, & Fauth, 1997; Wilson & Brekke, 1994).

We tend to ignore the importance of having an adequately large sample size when we function as intuitive scientists. This is why testimonials are so very powerful in persuading people what to do and believe. However, testimonials are based on the experiences of only one person, and often that person is being paid to say that some product or purchase is good. I have an advertisement for a psychic phone line that offers "real cases" as proof that psychics are

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