When Math Lies: 5 Ways your Performance Report can Mislead

How did you feel when you opened your most recent quarterly performance report from Schwab or your 401(K)? More importantly, have you ever made regretful investment decisions based on that emotional reaction? In periods of big market moves, up or down, we can lose perspective and abandon reliable long-term strategies. The numbers on a performance report are very good at deceiving us.

The calculations on a performance report are mathematically correct. Many of the conclusions we instinctively draw from them are not. Here are five reasons why.

1. The wrong yardstick

"My portfolio is up 10 percent, but the S&P is up 15. Why am I lagging?"

Because your portfolio isn't the S&P 500 and was never supposed to be. A diversified portfolio — bonds, international stocks, real assets, cash — is built for a different job than an index of large US companies, and it will always trail whatever single asset class is winning at the moment. It is working as designed. So unless you are ready to give up on diversification, the better performance measure is to compare each asset class to its peers. US stocks perhaps to the S&P, fixed income holdings to the Bloomberg U.S. Aggregate Bond Index, etc. But even that can be misleading. Keep reading…

2. The endpoint illusion

Consider the data below showing performance of the Vanguard Total Stock Market index from two consecutive one-year periods. One ending near the bottom of the Covid market drop, and one beginning just as the tremendous post Covid bull market was beginning.

Window	Growth of $10,000	One-Year Return
Apr 2019 – Mar 2020	$9,073	-9.3%
Apr 2020 – Mar 2021	$16,290	+62.9%

Vanguard Total Stock Market ETF. Source: Portfolio Visualizer.

The extreme distance between the two measurements makes sense in hindsight but don’t discount the intensity and emotion present at the time. Whether you owned this particular holding or not, reviewing your performance in those periods likely played heavily on your natural fears (Covid is collapsing the economy!) or greed (I’m an investment genius!). But wild swings are the story of the stock markets, and a one-year return is one frame in a long-running film. It repeated in miniature last year: measure a portfolio March-to-March across the spring 2025 tariff selloff and you get one story; measure April-to-April, starting at the bottom, and you get a dramatically rosier one.

By the way, fund marketers understand this mechanic thoroughly. It is no coincidence that advertising clusters in the months after a bad quarter rolls out of the trailing figures. The defense is simple: keep the context of time in mind. When the reporting time frame includes large market events, or excludes them, expand the time horizon to get a fuller view of relative performance.

3. Luck hiding inside the averages

In 2020, the Cathie Wood led ARK Innovation ETF (ARKK) returned more than 150 percent — a genuine number, produced by a once-in-a-generation liquidity surge meeting a concentrated portfolio of exactly the stocks that surge favored.

Here's what is easy to miss: an outlier year doesn't just distort that year. It contaminates every trailing average it touches. At the end of 2021 — a year in which ARKK lost roughly a quarter of its value — its trailing five-year figure still showed on the order of +38 percent annualized, because the 2020 spike sat inside the window. The fact sheet looked spectacular at precisely the moment the fund was collapsing. Investors responded to the math: ARK funds attracted an estimated $29 billion in 2020 and 2021, with assets peaking in mid-2021 — just as performance rolled over. The fund went on to lose 67 percent in 2022, and Morningstar later ranked the ARK family the industry's leading destroyer of shareholder wealth over the decade, at more than $14 billion.

The same mechanism operates at smaller scale everywhere: the value fund that happened to hold a meme stock, the sector fund that owned an acquisition target, the bond fund with one lucky duration call. Yet so often investors, including professionals, see that “track record” and pile in the fund expecting a repeat of the improbable.

4. Your report versus their report

Your personal performance report produced by the custodian or advisory firm, to its credit, measures the right thing: your dollars, over your actual time frame. The damage starts when you set that number beside one measured over a different window — a fund fact sheet, an index return from the news, a friend's bragging at dinner. Those comparisons are almost always apples to oranges.

I see this in my own practice. To this day, clients who happened to move their accounts to my care in April of 2020 — right after the Covid correction — show since-inception returns superior to clients who arrived a year later. The difference is the inception date and the comparison is misleading.

There's a second layer: fund fact sheets report time-weighted returns, which deliberately ignore when money entered or left. Your outcome depends enormously on exactly that. In ARKK's case, Morningstar estimated shareholders' dollar-weighted losses ran more than double the fund's reported figures, because most of the money arrived after the big returns were already gone. The fund's number and its investors' experience pointed in opposite directions — and both were accurate.

5. The rearview mirror

Every disclosure carries the boilerplate: past performance is not indicative of future results. It is such a ubiquitous warning we ignore it like the safety card in the seatback pocket. The data suggests we should pay attention. Extraordinary trailing performance, good or bad, is often inversely related to future performance.

Consider semiconductors. As of early July, the VanEck Semiconductor ETF is up roughly 69 percent this year alone — against about 7 percent for the S&P 500 — up more than 115 percent over the trailing twelve months, and compounding at roughly 63 percent annualized over three years. Those numbers are accurate. Now run the logic forward: at that pace, money doubles about every fifteen months. Sustained for a decade, twenty-five chip companies would be worth several times today's entire global stock market. Unlikely. It’s more likely the spectacular trailing return means much of the future's return has already been pulled into the past you're admiring.

This is a tendency, not a law — terrible past performance isn't a buy signal either. But the S&P Persistence Scorecard has documented it for two decades: top-quartile funds rarely stay top-quartile. The feeling that a big trailing number is evidence is backwards more often than investors want to believe.

What the report can't tell you

The report isn't lying. It's answering narrow questions with great precision while the questions that determine your outcome go unasked:

My favorite questions and the ones I suggest you ask yourself or your advisor? “Why do I own the assets I own?” and “Do we expect that portfolio to deliver the needed return in the needed timeframe for my goals?”

And of course, “Am I on track to meet those goals?”

Performance reporting is an important tool but beware of how it might mislead and manipulate you as an investor.