Black-Scholes assumes options are held to expiry and never forfeited. Real employee options are different: holders exercise early when the option is sufficiently in-the-money, and a meaningful fraction of options are forfeited each year as employees leave before vesting. The CRR binomial lattice prices both behaviors explicitly — producing a fair value that is typically lower than Black-Scholes by 10-25%, and that is the number an auditor will accept for awards where early exercise + turnover are documented.
Empirically observed early-exercise behavior and annual employee turnover. SEC guidance accepts both inputs when supported by historical data.
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