Kurtosis
Also called: excess kurtosis, leptokurtic, fourth moment
Kurtosis measures how much of a distribution's variance comes from rare extreme observations rather than ordinary ones. A normal distribution has kurtosis of 3, and financial returns routinely show far more — the statistical fingerprint of fat tails. High kurtosis is why models calibrated on typical days understate crisis losses: the average is unchanged, but the probability of a five-sigma move is orders of magnitude higher than the bell curve allows.
Where this is taught
Definitions are the trailer. These free levels turn Kurtosis into something you play — one bite-size lesson, with worked examples, a quiz and XP.