Path-Dependent Options
- Some payoffs depend on the whole history of the stock price, not only on S_{T}.
- The closed-form prices above work because each payoff depends only on S_{T}, whose risk-neutral distribution is lognormal.
- A binomial tree handles path dependence directly, as long as we follow each path separately.
- The stock tree still recombines, but the path information does not: two paths that end at the same price can have different maxima or averages.
- A tree with n steps has n + 1 final prices but 2^{n} paths, over a million for n = 20.
- In practice: record at each node what the payoff needs (running maximum, barrier hit), or simulate risk-neutral paths and average the discounted payoffs.
One Path, Its Extremes and Its Average
![A simulated stock price path over one year starting at 100, falling to a minimum of 81.75 after mid-year and rising to a maximum of 121.13 near month eleven, ending at 117.75. The running maximum steps up to 121.13, the running minimum steps down to 81.75, and the running average stays near 100.]()
One simulated year of daily stock prices starting at S_{0} = 100 with \sigma = 0.30, with the running maximum, minimum, and average.
Lookback Options
- A floating lookback call pays S_{T} - S_{\textit{min}}: buy at the lowest price the stock reaches.
- A floating lookback put pays S_{\textit{max}} - S_{T}: sell at the highest price the stock reaches.
- A fixed lookback call pays \max(S_{\textit{max}} - K, 0).
- A fixed lookback put pays \max(K - S_{\textit{min}}, 0).
- S_{\textit{max}} and S_{\textit{min}} are taken over the whole life of the option, including S_{0}.
- Analytic formulas exist for all four types.
Asian Options
- The payoff depends on the average stock price \bar{S}_{T} from time 0 until T; on a tree, the arithmetic mean of the prices along the path, including t = 0.
- Average price: call \max(\bar{S}_{T} - K, 0), put \max(K - \bar{S}_{T}, 0).
- Average strike: call \max(S_{T} - \bar{S}_{T}, 0), put \max(\bar{S}_{T} - S_{T}, 0).
- Averaging smooths the path, so an average price option is usually cheaper than the European one: popular with firms that buy or sell regularly over a period.
- No exact analytic formula for the arithmetic average (options on the geometric average have one); approximations assume the average is lognormal.
Example 6
- A non-dividend paying stock trades for $100 and every 3 months can rise or fall by 5%. The risk-free rate is 6%.
- We price a floating lookback put paying S_{\textit{max}} - S_{T} and an average price call paying \max(\bar{S}_{T} - 100, 0), both in 6 months.
Example 6 (cont’d)
- The risk-neutral probability of an up-move is
q = \frac{e^{0.06 \times 3/12} - 0.95}{1.05 - 0.95} = 0.6511
- Working backward, the lookback put L and the Asian call A are worth
\begin{aligned}
L_{u} & = (0 q + 5.25 (1 - q)) e^{-0.06 \times 3/12} = 1.80 \\
L_{d} & = (0.25 q + 9.75 (1 - q)) e^{-0.06 \times 3/12} = 3.51 \\
L & = (1.80 q + 3.51 (1 - q)) e^{-0.06 \times 3/12} = 2.36 \\[4pt]
A_{u} & = (5.08 q + 1.58 (1 - q)) e^{-0.06 \times 3/12} = 3.80 \\
A_{d} & = 0 \\
A & = (3.80 q + 0 (1 - q)) e^{-0.06 \times 3/12} = 2.44
\end{aligned}
Barrier Options
- Calls or puts that are activated or deactivated when the stock hits a barrier.
- In options come into existence only if the stock hits the barrier before maturity; out options die if it does.
- Up options require the stock to hit the barrier from below, down options from above.
- Eight combinations: up/down, in/out, call/put.
- An in and an out option with the same direction, strike, barrier, and maturity add up to the plain European option.
- A binomial tree tracks, along each path, whether the barrier has been hit; analytic formulas also exist.
In-Out Parity
![Option value against the barrier H from 60 to 100. A dashed line at 14.23 is the European call. The down-and-out call starts near 14.23 for low barriers and falls to zero at 100; the down-and-in call mirrors it, rising from zero to 14.23. The two add up to the dashed line at every barrier.]()
Black-Scholes prices of a one-year down-and-in and down-and-out call against the barrier H, with S = K = 100, r = 5\% and \sigma = 30\%. They add up to the European call, dashed.
Example 7
- A down-and-in call with barrier $90, strike $110, and maturity 6 months, on a stock at $100.
- The stock falls to $85 and finishes at $120: the call pays 120 - 110 = \$10.
- The stock never goes below $90 and finishes at $140: the call pays nothing, since it was never activated.
- The stock falls to $85 and finishes at $105: the call is activated but out-of-the-money, and pays nothing.
Barrier Options in Practice
- Monitoring. The contract states how often the barrier is checked: continuously, or at specified times such as daily closes.
- A barrier checked less often is hit less often: discrete monitoring raises knock-out prices and lowers knock-in prices.
- A tree checks the barrier only at its nodes, so it needs many steps to price a continuously monitored barrier.
- Hedging. Near the barrier of an up-and-out call, a small rise in the stock can wipe out a large payoff.
- The value falls steeply and delta is large and negative.
- Cash-or-nothing options have the same problem near the strike close to maturity.
- A delta hedger must trade large amounts of stock for small price moves.
Hedging Near the Barrier
![Option value against the stock price from 60 to 130 with a vertical line at the barrier. With one year left the value stays below 2. With three months it peaks near 5 at 110 and falls to zero at 130. With one week it tracks the payoff up to about 18 at 120, then drops steeply to zero at the barrier.]()
Black-Scholes prices of an up-and-out call with K = 100 and H = 130 for three maturities, with r = 5\% and \sigma = 30\%. The dotted line is the call payoff if never knocked out.