Unlock: Lognormal Distribution

A random variable is Lognormal if its logarithm is Normal. The density, mean, variance, median, and mode all have closed forms in the two underlying Normal parameters. The Lognormal is the multiplicative analogue of the Normal: a product of many independent positive factors is approximately Lognormal in the same way a sum is approximately Normal. Applications cover financial returns (with the heavy-tail caveat that real returns are heavier than Lognormal), particle sizes, lifetimes, and insurance severity.