ARITHMETIC RANDOM WALKS
Instead of assuming that at each step the asset price can only move up or down by a certain multiple with a given probability, we could assume that the price moves by an amount that follows a normal distribution with mean μ and standard deviation σ. In other words, the price for each period is determined from the price of the previous period by the equation
where

is a normal random variable with mean 0 and standard deviation σ. We will also assume that the random variable

, describing the change in the price in one time period is independent of the random variables describing the change in the price in any other time period.
80 A sequence of independent and identically distributed (IID) random variables

,... with zero mean and finite variance σ
78 is sometimes referred to as
white noise.
The movement of the price expressed through the equation above is called an arithmetic random ...