Summary
We have seen the three major reference models used with the Black-Litterman model. These
reference models arise from very different ways of viewing the problem of estimating asset
returns.
The Canonical Reference Model is developed from Theil's Mixed-Estimation Model and
includes investor uncertainty in the estimates. The posterior estimates include an updated
covariance matrix. This model requires the investor to estimate an additional parameter τ which
impacts the posterior covariance matrix as well as the estimated returns.
The Hybrid Reference Model was developed before the Alternative Reference Model, but any
formulation using it may be replicated in the Alternative Reference Model.
The Alternative Reference Model is a shrinkage model using some of the formulas from the
Black-Litterman model. It is simpler than the Original Reference Model because it does not
require the parameter τ, and requires the investor to only estimate one parameter to adjust the
shrinkage. It does not have a clear theoretical footing.