Alpha

Alpha Explained

Return generated above and beyond what the market explains

Formula

Rp − [Rf + β(Rm − Rf)]

portfolio return minus the expected return predicted by CAPM — i.e., the risk-free rate plus Beta times the market risk premium.

What is the Alpha?

Alpha (Jensen's Alpha) measures the excess return your portfolio generates above what the Capital Asset Pricing Model (CAPM) would predict given its level of market risk (Beta). A positive Alpha means you're outperforming on a risk-adjusted basis — your stock selection or timing has added value above and beyond what market exposure alone would explain. Negative Alpha means you're underperforming the risk-adjusted benchmark.

How to interpret it

Alpha is expressed as a percentage return. An Alpha of +3% means your portfolio returned 3% more per year than CAPM would predict given its Beta. This sounds attractive, but statistical significance requires a long track record — a few months of positive Alpha may be luck. Alpha is most meaningful over 3–5 years with high R². A high-Alpha, high-R² portfolio is genuinely adding value above market exposure.

What counts as a good Alpha?

< −2%Significant underperformance — fees, poor timing, or selection are hurting returns

−2% to 0%Slightly below benchmark on a risk-adjusted basis

0% to 2%Marginal outperformance — could be skill or short-term luck

> 2%Meaningful outperformance — significant if sustained over 2+ years with high R²

What affects your Alpha?

Stock selection — picking assets that outperform their risk-adjusted expectation
Market timing — entering and exiting positions well
Fees and costs — transaction costs directly erode Alpha
Beta accuracy — a poorly calibrated Beta distorts the Alpha calculation
Benchmark choice — changing the benchmark changes Alpha significantly

How Portivex uses Alpha

Portivex surfaces Alpha as part of the advanced benchmark metrics alongside R² and Beta. Because Alpha requires sufficient R² to be meaningful, Portivex flags Alpha when R² is below 0.5 — indicating the CAPM model isn't fitting your portfolio well enough for Alpha to be reliable. Like all metrics, confidence tiers apply.

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Frequently asked questions

Can retail investors consistently generate positive Alpha?

Academic research suggests it's extremely difficult over the long run. Most actively managed funds fail to beat their benchmark after fees. Retail investors face additional headwinds: higher per-trade costs, less access to information, and behavioural biases. Positive Alpha over 1–2 years is likely noise; over 5+ years with high R², it's worth paying attention to.

Why does my Alpha change when I change the benchmark?

Because Alpha is defined as the return above what the chosen benchmark-and-Beta model predicts. A portfolio that looks like +5% Alpha against a UK small-cap benchmark might show −2% Alpha against the MSCI World. Always specify the benchmark when quoting Alpha.

Is Alpha the same as outperformance?

Not exactly. Simple outperformance is just 'my portfolio returned more than the index'. Alpha is risk-adjusted outperformance — it strips out the contribution of market Beta. A portfolio that returned 20% in a year when the market returned 18% looks like 2% outperformance. But if that portfolio had a Beta of 1.3 and the market was up 18%, CAPM would expect it to return 23.4% — meaning the Alpha is actually negative.

Related metrics

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