The Intelligent Investor’s Guide to Option Pricing  •  59
the size of the BSM’s probability cone itself. When the cone changes size, 
the range-of-exposure area within the cone also changes. Let’s explore this 
concept more.
How Changing Market Conditions 
Affect Option Prices
At the beginning of Chapter 2, I started with an intuitive example related 
to a friendly bet on whether a couple would make it to a restaurant in time 
for a dinner reservation. Let’s go back to that example now and see how the 
inputs translate into the case of stock options.
Dinner Reservation Example Stock Option Equivalent
How long before seating time Tenor 3 of the option
Distance between home and restaurant Difference between strike price and 
present market price (i.e., strike–stock 
price ratio)
Amount of traffic/likelihood of getting caught 
at a stoplight
How much the stock returns are 
thought likely to vary up and down 
Average traveling speed Stock market drift
Gas expenditure Dividend payout
Looking at these inputs, it is clear that the only input that is not known 
with certainty when we start for the restaurant is the amount of traffic/
number of stoplights measure.
Similarly, when the BSM is figuring a range of future stock prices, 
the one input factor that is unknowable and that must be estimated is 
how much the stock will vary over the time of the option contract. It is 
no surprise, then, that expectations regarding this variable become the 
single most important factor for determining the price of an option and 
the factor that people talk most about when they talk about options—
volatility (vol). 
This factor is properly known as forward volatility  and is formally 
defined as the expected one-standard-deviation fluctuation up and 
down around the forward stock price. If this definition sounds familiar,