Tracking error

Many portfolios are managed to a benchmark, such as an index (i.e. the S&P500 in the USA or the CAC40 in France). Some portfolios are expected to replicate the returns of an index exactly (an index fund), while others are expected to deviate slightly from the index in order to generate excess returns or to lower transaction costs. Tracking error (a.k.a. Active Risk) is a measure of how closely the portfolio follows the index, and is measured as the standard deviation of the difference between the portfolio and index returns.

There is two ways to express the objective of a fund manager: either minimizing the tracking error for a given expected return over a predefined benchmark or maximizing the expected return for a given tracking error.

Ex-ante vs ex-post tracking error

If tracking error is measured historically, it is called 'realised' or 'ex post' tracking error. If a model is used to predict tracking error, it is called 'ex ante' tracking error. The former is more useful for reporting or analysis purposes, whereas ex ante is generally used by portfolio managers to control risk to satisfy client guidelines.

Hwang and Satchell (2001) have argued that ex-ante and ex-post tracking error must differ, as portfolio weights are ex-post stochatic in nature. Futhermore, they showed that ex-port tracking error must be higher than ex-ante tracking error.

Mathematical definition

As defined in Chincarini and Daehwan (2006), most portfolio managers when using tracking error define it as beeing the standard deviation of the returns of the portfolio minus the returns of the benchmark.

It can be expressed as:

$TE=\sigma (r_{P}-r_{B})={\sqrt {r_{P}-r_{B}}}$

where $r_{P}$ is the returns of the portfolio $r_{B}$ is the return of the benchmark

References

Chincarini, L. and Daehwan K. (2006), Quantitative Equity Portfolio Management, Mc Graw Hill

Satchell, SE and Hwang, S, "Tracking error: Ex ante versus ex post measures", Journal of Asset Management, Volume 2, Number 3, 1 December 2001, pp. 241-246(6)