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** Contents**
** Index**

From a statistical point of view, feature selection
is problematic. For a test with one degree of freedom, the
so-called Yates correction should be used (see
Section 13.7 ), which makes it harder to reach
statistical significance. Also, whenever a statistical test
is used multiple times, then the probability of getting at
least one error increases. If 1000 hypotheses are rejected,
each with 0.05 error probability, then
calls of the test will be
wrong on average. However, in text classification it rarely
matters whether a few additional terms are added to the
feature set or removed from it. Rather, the *relative*
importance of features is important.
As long as feature selection
only ranks features with respect to their usefulness and is
not used to make statements about statistical dependence or
independence of variables, we need not be overly concerned
that it does not adhere strictly to statistical theory.

© 2008 Cambridge University Press

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2008-06-01