The objective of span is to identify overall risk in a portfolio of futures and options contracts for each member.
In standard pricing models, three factors most directly affect the value of an option at a given point in time:
- Underlying market price
- Volatility (variability) of underlying instrument
- Time to expiration
As these factors change, so too will the value of futures and options maintained within the portfolio. Span constructs scenarios of probable changes in underlying prices and volatilities in order to identify the largest loss a portfolio might suffer from one day to the next. It then sets the margin(capital) requirement at a level sufficient to cover this one-day loss.