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Most
financial
engineering models are what are known as
relative pricing models.
They price instruments based on prices of other instruments quoted in the
market—an instrument's price is determined relative to other prices
quoted in the market.
Financial theorists and
economists often use what are known as
absolute pricing models.
These price instruments without regard for other prices quoted in the
market. Instead, they base prices on economic theory,
assumptions about the preferences of economic agents, supply and demand, etc.
Most absolute pricing
models are what are known as
equilibrium pricing models.
They calculate at what prices a market will reach equilibrium—where
supply and demand balance and the market clears. A
shortcoming of many equilibrium models is the fact that they cannot be
calibrated to current market prices. They calculate hypothetical
equilibrium prices that generally don't match actual prices currently
observed in the market. Because this violates the
law of one price, such models are
useless in a trading context. Equilibrium models are largely theoretical
tools. Sharpe's
capital asset pricing model
is an equilibrium pricing model.
Most relative pricing models employed by financial
engineers are based on the theory of
arbitrage-free pricing. Prices are determined relative to other prices
quoted in the market in such a manner as to preclude any
arbitrage
opportunities.
An arbitrage condition
is a relationship that must prevail between certain prices if they are to
be arbitrage-free. Examples of arbitrage conditions are:
interest rate
parity for forward exchange rates;
put-call parity for
European options;
cash-and-carry arbitrage conditions for forward commodity prices.
With arbitrage-free pricing, financial engineers apply arbitrage conditions to
prices that are observable in the market in order to determine other
prices that are not. Standard
formulas for pricing forwards, swaps and debt instruments are all
derived using such arbitrage arguments. In complete markets,
arbitrage-free pricing can be used to uniquely determine a price for any
instrument. In incomplete markets, it may only place bounds on certain
prices.
The groundbreaking Black-Scholes (1973) approach to
pricing options is based on arbitrage-free pricing. Black and Scholes
identified an arbitrage condition that, given certain simplifying
assumptions, must hold between the price of an
option and the value of a
corresponding replicating portfolio. Based upon this, they were able to
price options. That same approach, modified in many different ways,
underlies essentially all models used today for pricing options and other
derivative instruments in complete markets. In essence, much of financial engineering is little more
than aggressive and creative use of arbitrage-free pricing.

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