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Building Machine Learning Systems with Python by Willi Richert, Luis Pedro Coelho

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P greater than N scenarios

The title of this section is a bit of inside jargon, which you will now learn. Starting in the 1990s, first in the biomedical domain and then on the Web, problems started to appear when P was greater than N. What this means is that the number of features, P, was greater than the number of examples, N (these letters were the conventional statistical shorthand for these concepts). These became known as "P greater than N" problems.

For example, if your input is a set of written text, a simple way to approach it is to consider each possible word in the dictionary as a feature and regress on those (we will later work on one such problem ourselves). In the English language, you have over 20,000 words (this is if you perform ...

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