Consider the continuous compound interest formula
Where is the total amount after time
when the amount
was invested at time 0 at an interest rate
. So,
is the rate of change of
, i.e.
Thus, the differential equation for this change is given as
or
Where represents change in the amount
and
represents change in time
. And,
is called drift coefficient (or drift rate) that is the growth rate of the amount
.
In general, is used to represent an asset price, and
is used to represent drift coefficient. So, by replacing
with
and
with
, the above equation is given by
This is the case when we do not have any uncertainty, for example, we invested the amount or asset in a bank that pay fixed interest rate. For an asset price or a stock price, of course, there is uncertainty. The price may go up or down any time in the future. This uncertainty (or standard deviation) is represent by . Like drift coefficient
that is in relation with the asset price
, i.e.
, the uncertainty
is also given in relation with the asset price
, i.e.
. Besides, this uncertainty is assumed to follow Brownian motion
. The above differential equation is now
This differential equation has a stochastic variable that changes randomly. Hence, it is called stochastic differential equation. Or any differential equation that has at least one stochastic variable is called stochastic differential equation.