What Is the Security Market Line?
Let me explain the security market line, or SML, directly to you. It's a visual tool that plots different levels of systematic risk for various marketable securities against the expected return of the entire market at any point in time. You might also hear it called the 'characteristic line,' and it's essentially a graph of the capital asset pricing model, or CAPM. On this chart, the x-axis represents risk in terms of beta, and the y-axis shows expected return.
The market risk premium for any given security comes from its position on this chart relative to the SML itself. That's how you gauge it.
Key Takeaways
Here's what you need to know about the SML. It can help you figure out if an investment product offers a favorable expected return given its risk level. The formula to plot it is straightforward: required return equals the risk-free rate of return plus beta times (market return minus risk-free rate of return). Money managers and investors like you commonly use the SML to evaluate potential investments.
Understanding the Security Market Line
I want you to understand that the SML comes from the CAPM, which models the risk-return relationship for securities. It's based on the idea that investors, including you, need compensation for the time value of money and the risk involved in any investment—that's the risk premium. By looking at an SML graph, you can interpret market risks more clearly.
Beta is key here in both CAPM and SML. It measures a security's systematic risk, the kind you can't diversify away. A beta of one matches the market average. If it's greater than one, the risk is above average; less than one, it's below. Remember the plotting formula: required return = risk-free rate of return + beta (market return - risk-free rate of return).
One important note: while the SML is useful for evaluating and comparing securities, don't rely on it alone. The expected return over the risk-free rate isn't the only factor when choosing investments.
Using the Security Market Line
You can use the SML to evaluate investment products you're considering for your portfolio. It shows whether a security offers a good expected return for its risk level. Plot a security on the chart—if it's above the SML, it's undervalued because it promises more return than its risk suggests. If it's below, it's overvalued since the return doesn't justify the risk.
The SML is great for comparing two similar securities with about the same return to see which has less inherent market risk. Or, use it to compare securities with equal risk and find which one gives the highest expected return for that risk.
