Thinking and Deciding

Chapter 7
Hypothesis testing
Data! Data! Data! I cannot make bricks without clay.
Sherlock Holmes
A hypothesis is a proposition that we evaluate, or test, by gathering evidence concerning
its truth or, more generally, its probability. A physician trying to explain
a patient’s illness forms hypotheses about the patient’s disease and then tests these
hypotheses by asking the patient questions and ordering various tests. A scientist
trying to explain the cause of a disease also forms hypotheses and tests them by doing
experiments or by gathering other sorts of evidence (such as statistics about the
incidence of the disease).
Hypothesis testing, the topic of this chapter, is the selection of the evidence, the
asking of questions. The physician must decide what questions to ask, and the scientist
must decide which data to collect. Hypothesis testing is that part of the searchinference
process in which the thinker searches for evidence that can strengthen or
weaken various possibilities. Each possibility is a possible answer to some question
(goal) that inspired the search for hypotheses: What is wrong with the patient? What
causes this disease? 1
Hypothesis testing is important in everyday life and in other professions as well
as in medicine and science. Children may learn the meanings of words by testing hypotheses.
When we solve a test problem from Raven’s Progressive Matrices (Chapter
2), we gather evidence to test some hypothesis about the rule. We try out a command
on a computer in order to test some hypothesis about what the command does. Clinical
psychologists test hypotheses in the same way physicians do, and, of course,
detectives test hypotheses all the time.
Hypothesis testing is an essential part of actively open-minded thinking because
it involves putting our beliefs to the test of evidence. The very act of testing a favorite
hypothesis requires that we be open to the possibility that it is incorrect (if we are
1The term “hypothesis testing” is used in statistics to refer to making inferences from data. That is a
different question.
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