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115 CFA
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115.020.40.10 Reading 9 - 10. The Lognormal Distribution

# 10. The Lognormal Distribution

## o. explain the relationship between normal and lognormal distributions and why the lognormal distribution is used to model asset prices;

What is a lognormal distribution?
A random variable, Y, follows a lognormal distribution if its natural logarithm, lnY, is normally distributed. You can think of the term lognormal as "the log is normal." For example, suppose `X` is a normal random variable, and `Y = e^X`. Therefore, `Ln(Y) = Ln(e^X) = X`. Because X is normally distributed, Y follows a lognormal distribution.

Just like the normal distribution, what two parameters completely describe a lognormal distribution?
Mean and variance can completely describe both a normal and lognormal distribution. However, unlike a normal distribution the mean and variance for a lognormal distribution are not identical.

What is the lower bound of a lognormal distribution?
Zero

Which direction does a lognormal distribution skew?
Always to the right. It is bounded at 0 and skews to the right

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