# blacklitterman.org

## Cookbook

My paper on the Black-Litterman Model (Updated 20 June 2014), Accompanying MATLAB codes also on the site

A new spreadsheet which illustrates the differences between the reference models.

A new paper Reconstructing Black-Litterman is now available at SSRN. This paper offers a critique of Michaud et al's recent paper, Deconstructing the Black-Litterman Model, from the Journal of Investment Management.

The author's methods section has been updated with a new taxonomy of the model, and many papers have been added.

A new implementation of the Black-Litterman model in Excel is available on the implementations page.

An implementation of the Black-Litterman model in python and the worked example from the He and Litterman 1999 paper (Updated Jun 22 2012)

An excel spreadsheet showing the example worked in the He and Litterman paper (Updated Jun 26 2012)

New paper focusing on Tau and if you really need it (Updated 1 November 2010)

MATLAB and SciLAB implementations of the model

An applet which implements the Black-Litterman model

Here are the core Black-Litterman formulas as used in the canonical expression of the Black-Litterman model. The investor is uncertain in their estimates (prior and views), and expresses them as distributions of the unknown mean about the estimated mean. As a result, the posterior estimate is also a distribution.

Note that there are authors who do not use the canonical expression of the model. You can track down which authors use which expression of the model at author's methods.

Given the following inputs

 w Equilibrium market capitalization weights for each asset. Σ Matrix of covariances between the assets. Usually computed from historical data. rf Risk free rate δ The risk aversion coeffficient of the market portfolio. This can be specified, or can be computed if the investor knows the market return and standard deviation of returns. τ A measure of the uncertainty of the prior estimate of the mean returns.

First we use reverse optimization to compute the vector of equilibrium returns, Π, using the following equation.

At this point the investor needs to quantify their uncertainty in the prior by selecting a value for τ. If the covariance matrix has been generated from historical data, then τ = 1/n is a good place to start.

Next the investor formulates their views, specifying P, Ω, and Q. Given k views and n assets, then P is a k × n matrix where each row sums to 0 (relative view) or 1 (absolute view). Q is a k × 1 vector of the excess returns for each view. Ω is a diagonal k × k matrix of the variance of the estimated view mean about the unknown view mean. As a starting point, some authors call for the diagonal values of Ω to be set equal to pTτΣp (where p is the row from P for the specific view). This weights the views the same as the prior estimates.

Next, we apply the Black-Litterman 'master equation' to compute the posterior estimate of the returns using the following equation.

Then we must compute the posterior variance of the estimated mean about the unknown mean using the following equation.

Closely followed by the computation of the variance of returns about the estimated mean. Note that because we have assumed the uncertainty in the estimates is independent of the known covariance of returns about the unknown mean, that we can just add the two together to get the covariance of retujrns about the estimated mean.

And now we can compute the portfolio weights for the optimal portfolio on the unconstrained efficient frontier. The Black-Litterman model does not require a specific method for portfolio choice. Here we use unconstrained mean variance optimization because it is very easy to understand, but you can add constraints or use another objective function such as CVAR.

Implementations in SciLab and MATLAB are available from here. For a more thorough discussion see my paper on the Black-Litterman model.

©2000-2013 Jay Walters. The opinions expressed on this website are my own and not those of my employer.

This website is provided "as is" without any representations or warranties, expres or implied. All content provided on this site is for informational purposes only.