What Is Overfitting, in Plain Words

Overfitting in five lines

Overfitting, in plain words, is a rule that has learned the accidents of its bars along with whatever pattern was in them. It happens when many candidate rules are tried on few bars and the best is kept, because the best of many candidates on few bars is mostly the luckiest, not the truest. It looks like a perfect past and a poor next week, and nothing in the past can tell you which you have. The cure is fewer knobs to turn, more bars to turn them on, and one look at bars set aside, taken once. In the example, the five best buy-and-sell hour-pairs on the euro for 17 to 21 August 2026 all lost money on the week that followed.

A jacket tailored to one day

Picture a jacket tailored to one person on one day. It was measured on a Wednesday after a large lunch, and it is perfect on that Wednesday; every other day it is tight across the middle, because the measurements included an accident of the day and the tailor cut to them. A good tailor allows for the days that were not measured. An overfitted rule is the jacket cut to the inch: the tighter it fits the bars it was measured on, the fewer other weeks it will fit, and the perfection is the problem, not the proof.

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