What it means
Michael Jensen introduced the measure in the 1960s to judge mutual fund managers fairly. Comparing raw returns is misleading, because a fund that took big risks should be expected to earn more than a cautious one.
The measure is now a standard tool in performance reports from asset managers. The measure builds on the Capital Asset Pricing Model, or CAPM, which says the return you should expect depends on the risk-free rate (the return on a very safe investment) and on beta (how strongly the portfolio moves with the market).
Jensen's alpha is the gap between the actual return and that expected return. In plain terms, alpha is what is left after paying for the risk.
A positive alpha suggests the manager found returns that risk alone does not explain, perhaps through good stock selection. A negative alpha suggests the portfolio earned less than it should have for the risk taken, which often happens once fees are deducted.
Jensen's alpha is closely related to the idea of an efficient market, where it is hard to beat the market consistently. If most managers show an alpha close to zero or below after costs, that supports the case for low-cost index funds.
Investors who still pay for active management should ask for evidence of a persistent positive alpha. Managers and investment committees use the measure to compare funds and to decide whether an active manager justifies their fee.
It works best when comparing portfolios against a suitable benchmark over a reasonably long period, since a single year can be luck. A manager with a modest but steady alpha over ten years is usually more convincing than one with a big alpha in a single year.
There are limits. Alpha depends on the beta and the benchmark chosen, so a poor benchmark gives a misleading answer.
It also assumes that CAPM is a good model of expected returns, which is debated.
In practice
Real-world examples.
Example
A family office reviews two equity funds that each returned 11%. Fund A had a beta of 0.8 and fund B a beta of 1.4, so the committee uses Jensen's alpha and finds that fund A added far more value for the risk taken. The committee decides to keep fund A and to ask fund B's manager to explain the extra risk.
Example
A pension trustee in the healthcare sector sees that a manager's alpha has been negative for five years. She uses the figure to question whether the manager's fees are justified. The committee gives the manager one more year and sets a clear alpha target before deciding whether to switch to a cheaper index fund.
Example
A small business owner who invests surplus cash through an adviser asks for the portfolio's alpha. The adviser explains that the 0.5% positive alpha shows a modest gain after adjusting for market risk. She decides to keep the portfolio but to review the alpha again after a further year of results.
Formula
Calculation
Jensen's alpha = Portfolio return - [Risk-free rate + Beta x (Market return - Risk-free rate)]
Suppose a fund returned 12% over the year. The risk-free rate was 3%, the market returned 9%, and the fund's beta was 1.2.
Expected return = 3% + 1.2 x (9% - 3%) = 3% + 1.2 x 6% = 3% + 7.2% = 10.2%
Jensen's alpha = 12% - 10.2% = 1.8%
The fund beat what its risk level predicted by 1.8 percentage points, which on a $500,000 portfolio is worth about $9,000 of extra return (1.8% x $500,000 = $9,000).Case study
Seen in the real world.
This is a fictional illustration. Northgate Capital, an invented investment firm, ran a growth fund that returned 15% in a year when the market returned 10% and the risk-free rate was 2%.
The marketing team boasted of the five-point outperformance. Analyst Tomas calculated the fund's beta at 1.5, so the expected return was 2% + 1.5 x 8% = 14%, which left an alpha of only 1%.
He explained that most of the outperformance came from taking extra risk, not from skill. The firm changed its marketing to report risk-adjusted results, which made its claims more credible. Clients responded well to the honesty, and several asked for the firm's alpha figures every quarter.
Watch out
Common mistakes.
- Treating a high return as proof of skill. Without adjusting for beta, a high return may only reflect high risk. Beta and alpha together tell the fuller story.
- Using the wrong benchmark. A mismatched index produces a misleading beta and a meaningless alpha. Always check that the index matches the fund's actual holdings.
- Judging a manager on one year of alpha. Short periods contain a lot of luck, so longer track records are more reliable. Many professionals look for at least three to five years of data.
Questions
People also ask.
What does a negative Jensen's alpha mean?
It means the portfolio earned less than its market risk would predict. It can also reflect the cost of fees.
How is Jensen's alpha different from the Sharpe ratio?
Jensen's alpha measures excess return relative to a benchmark through beta, while the Sharpe ratio measures return per unit of total volatility. Alpha also depends on a chosen benchmark, whereas the Sharpe ratio does not need one.
Can alpha be zero?
Yes, which means the portfolio earned exactly what its level of market risk would suggest. This is common for passive funds that simply track the market.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%