Rolling Returns: Why the Start Date Matters

When looking at how an investment has performed, the usual question is how much it returned over the last 3, 5 or 10 years.

The problem is that this number describes a single window. Moving the start date can change the result considerably.

This is where rolling returns come in.

Instead of looking at one fixed period, you calculate successive periods of the same length. In a 10-year rolling return, for example, you see the result for someone who started investing at every possible moment and stayed invested for ten years.

The goal is not to find a single return. It is to understand how results changed depending on when you got in.

Not every 10-year period was the same

The chart shows something important: a long horizon narrows some of the differences between periods, but it does not remove risk.

There were 10-year windows in which US large-cap stocks returned less than zero. Between November 2008 and October 2010, the 10-year return stayed below zero for 24 months in a row: anyone who had invested close to the peak of the tech bubble reached the end of a decade still showing a loss.

It was not the only time. Between 1938 and 1941, those who had started investing near the 1929 peak went through the same, and that is where the worst result of the whole series was recorded, close to −5% a year.

This matters because it contradicts an overly simple idea:

investing for 10 years does not guarantee a positive result.

Historically, longer periods made positive results in stocks more frequent, but there were still exceptions.

The start date can change the result completely

Two investors can buy the same asset, hold it for the same number of years and end up with different results.

The only difference may be when they started.

A significant fall at the beginning of a period can weigh on the return for several years. Likewise, when a particularly negative period drops out of the calculation window, the rolling return can improve quickly.

That does not necessarily mean the market changed suddenly. Sometimes it is simply a consequence of how the period is being measured.

This is why looking at a single fixed interval can give an incomplete picture of risk and return.

The horizon matters, but it does not remove risk

Time is one of the most important factors in an investor’s ability to take risk.

Over short horizons, stock returns can vary widely. As the horizon lengthens, the spread of results has historically tended to narrow.

But narrowing does not mean disappearing.

This distinction is essential. A long horizon can make it easier to get through negative periods, but it does not turn a risky investment into a certainty.

It is also why the time horizon should be tied to financial goals. Money needed within two years should not be treated the same way as capital meant for a retirement several decades away.

The link between horizon and risk is also explored in the article on risk tolerance.

Look at many periods, not just one

Rolling returns are useful because they show that past performance is not a single number.

They let you see how the same strategy behaved for investors who started at different moments, and they help you understand the range of possible outcomes.

We looked at a century of data, including the main crashes and recoveries, in The Enduring Case for Stocks.

At PortfolYou, the Backtester lets you see how a specific portfolio would have behaved across different historical periods.

Monte Carlo approaches the same question another way, simulating many future scenarios and estimating the probability that a plan reaches a given goal.

One tool looks at different paths that already happened. The other tests different paths that might happen.

In the end, the most useful question is not just “how much did it return?”.

It is also “over how long, and starting when?”

Adapted from “Retornos e tempo – uma relação de longo prazo”, originally published on the Future Proof blog.