Beta

Beta is a numerical estimate of how sensitive the return of an individual security or portfolio is to the return of the market as a whole. It answers a single question: when the index moves, how far, on average, does this security move with it? Whatever in the security's behaviour cannot be explained by the market lies outside what beta describes — that is the residual, security-specific part of its risk, unsystematic risk.

What the value is made of

Beta expresses how the fluctuations of the security and those of the index are linked: the numerator is their joint movement, covariance, and the denominator is the dispersion of the index itself. This gives two things on which the value depends directly. The first is correlation: the more loosely a security tracks the market, the closer its beta is to zero, no matter how sharply the security itself swings. The second is the ratio of the amplitudes: a security may track the index perfectly yet do so in a more muted or a more pronounced way, and beta will lower or raise the estimate accordingly.

This is also where its arbitrariness comes from. Beta is not a property of the company but a property of the "security–index" pair over a chosen window. Switch the index, lengthen or shorten the window, move from daily to weekly returns — and you get a different value. That is why betas from different sources are comparable only when they are calculated on the same basis.

What it looks like on a chart

Take a large, liquid stock and look at how it behaved over a year alongside the index:

The stretches where the line follows the contour of the market are what shape beta; the sharp isolated spikes on corporate news barely register in it — they go into the residual and into alpha. When such securities are combined, their betas add up, weighted by their shares, into portfolio beta, and it is this figure that is managed when a portfolio's market sensitivity is being adjusted.

Where beta misleads

The main fallacy is treating beta as a measure of risk in general. It captures only the part of the fluctuations that comes from the market; a security's volatility can be high while its beta is low, if the source of the swings is internal. The second is reading beta as a forecast. It describes a past window, and for a company whose debt load is changing, its sensitivity drifts along with its capital structure; to compare industry peers, analysts use unlevered beta. The third is applying beta to asymmetric moves: the link with the market is often different in declines and in rallies, while a single number averages both regimes into one.