Kurtosis
A measure of tail heaviness; leptokurtic distributions have fatter tails (more extreme observations) than the normal distribution.
The exam’s favorite trap is the kurtosis-vs-excess-kurtosis swap: a question reports “kurtosis = 4” and asks if the distribution is leptokurtic — the answer turns on the excess value, 4 − 3 = 1 > 0, so yes. The “tell” is any value near 3; that’s mesokurtic, normal-like. Watch for platykurtic distributions (excess kurtosis < 0, thinner tails) as the wrong-direction distractor. Both measures come from the fourth moment about the mean.
The classic confusion is kurtosis vs skewness: skewness measures asymmetry (which tail is longer), while kurtosis measures tail weight, not direction — both tails fatten together (the curriculum and most question banks also describe leptokurtic shapes as more peaked, though modern statistics treats kurtosis as a tail-extremity measure, not peakedness). Don’t conflate fat tails with high variance either: two series can share the same variance yet differ in kurtosis, the leptokurtic one carrying far more extreme-outcome risk. Memory hook: “lepto = leaping” tails jump out further; platy = “plateau,” flat and squat.
PlayPrepHQ study notes are written and reviewed against primary exam sources. How we create & review content →