Asking the Right Questions, A Guide to Critical Thinking, 8th Ed

How Good Is the Evidence 1? 109
We will revisit this fallacy in Chapter 9 when we discuss research evidence
and issues of sampling.
Personal Testimonials as Evidence
I saw a note on a service station wall stating:
'Jane did a wonderful job fixing the oil leak my car had. You should always take
your car to Jane to fix that engine problem you have."
This book looks great. On the back cover comments from readers say, "I could
not put this book down."
"All my friends are saying about the new toothpaste. I should buy same quality."
Commercials, ads for movies, recommendations on the backs of book
jackets, and "proofs" of the existence of the paranormal or other controversial
or extraordinary life events often try to persuade by using a special kind of
appeal to personal experience; they quote particular persons as saying that a
given idea or product is good or bad, or that extraordinary events have
occurred, based upon their personal experiences. Such quoted statements
serve as personal testimonials. You may have listened to personal testimonials
from college students when you chose your college.
How helpful is such evidence? Usually, it is not very helpful at all. In most
cases, we should pay little attention to personal testimonials until we find out
much more about the expertise, interests, values, and biases behind them. We
should be especially wary of each of the following problems with testimonials:
• Selectivity. People's experiences differ greatly. Those trying to persuade us
have usually carefully selected the testimony they use. We should always ask
the question, "What was the experience like for those whom we have not
heard from?" Also, the people who provide the testimonials have often been
selective in their attention, paying special attention to information that confirms
their beliefs and ignoring disconfirming information. Often, believing
is seeing! Our expectancies greatly influence how we experience events. If we
believe that aliens live among us, or that humans never really landed on the
moon, then we are more likely to see ambiguous images as aliens or as proof
of the government conspiracy regarding the moon landing.
• Personal interest. Many testimonials such as those used for books, movies,
and television products come from people who have something to gain
from their testimony. For example, drug companies often give doctors