Features
One way to classify an object or event is to measure its characteristic
properties or features. For example, to
classify a printed letter, it might be helpful to know its area and its
perimeter. We could measure its compactness by the ratio of its area to
the square of its perimeter. We could measure the degree to which it is
symmetric about a horizontal axis by comparing the area of its upper half
to the area of its lower half. (You can probably think of even better ways
to measure symmetry.)
Some properties might be particularly sensitive to important small differences.
For example, to distinguish the letter "D" from the letter "O",
we might measure the straightness of the left side, perhaps by computing
the ratio of the straight-line distance to the arc length. Clearly, the
features that are needed depend on the specific problem that one wants to
solve, and designing a good set of features is more of an art than a science.
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