Conversations on Twitter are not known for their civility. Still, it came as a surprise this week when, out of the blue, author Nassim Taleb launched a broadside against investor Clifford Asness, calling his work “crap” among other insults.

Asness wasted no time firing back, calling Taleb “very wrong and clearly both nuts and a world class terrible person.”

From there, the insults escalated—nasty, overrated, unoriginal, illogical, pretentious, emetic. That last one I had to look up in the dictionary, and those are just the words fit for print.

If you’re not familiar with them, Taleb is a retired Wall Street trader turned author. He is most well known for his book The Black Swan. Asness is the founder of AQR Capital Management, which runs several public and private investment funds. Both are well respected in the investment field. And, according to Asness, they used to be friendly.

So why the sudden vitriol? It started when Taleb questioned the track record of some AQR funds, then touted the superior performance of funds managed by Universa, the firm where he is affiliated. Over the course of several days, Taleb and Asness debated a number of points, but fundamentally, it was an argument about investment performance.

Of all the topics to debate, investment performance seems like one that ought to be cut and dried. Suppose Fund A gains 5%, and Fund B gains 10%. Didn’t Fund B deliver better performance? What is there to argue?

It turns out that investment performance is, unfortunately, somewhat in the eye of the beholder. Below are just a few of the ways in which performance measurements can be fuzzy:

• Choice of benchmark – One of the oldest tricks in the book for fund managers is to choose a benchmark for comparison that is easy to beat. Fund companies have been known, for example, to choose a conservative benchmark then fill their funds with the stocks of small, fast-growing companies. The result: By comparison, the fund manager looks like a star.
• Choice of time period – Measuring performance inherently requires a time period. And yet, the reality is that every investment (or almost every investment) has periods when it’s in favor and when it’s out of favor. Choose one time period, for example, and you could argue that international stocks are superior to domestic stocks. But choose another time period, and you’ll get the opposite result. The same is true of almost every investment.
• Limited data – Try for a minute to evaluate an index of emerging markets stocks, and you’ll quickly get stuck. Why? For the U.S. market, there is reliable data going all the way back to 1926. But the most common emerging markets index only started in 1998. So how can they be compared, if at all? It’s a judgment call and certainly subject to debate.
• A changing world – The world is not a static place. Since 1926, for example, the U.S. stock market has returned about 10% per year, on average. But today our population is growing at just half the rate it was in decades past. So is that historical average meaningful or misleading? You could make the same observation about other countries, industries or companies. In every investment category, it’s very hard to make comparisons across time.
• Taxes – For individual investors, one of the more maddening aspects of performance reporting is that taxes are often ignored. If one fund manager is pursuing a tax-efficient strategy while the other is not, it’s very hard to compare them. That’s because tax rates differ across individuals. Those in the lowest tax brackets, in fact, pay zero capital gains taxes. That’s why fund managers don’t even try to report after-tax performance, even though that’s what really matters to each shareholder.

Asness summed it up best. In one of his tweets, he accused Taleb of comparing “apples to hippopotamuses.” And that is the heart of the problem. In this case, both Asness and Taleb have a vested interest in defending their own funds’ performance. But even for the individual investor trying to make objective judgments, it’s not easy. That’s why I recommend these basic steps as you build and monitor your portfolio:

• Simple – There are two reasons to favor investments that are simple and straightforward. The first is that they’ll usually have long track records. The second is that it will be easier to choose an appropriate benchmark. A big part of the Asness-Taleb debate is that they’re arguing over complex investments with track records that go back to only 2017 or 2018. Even if they could agree on a benchmark, a few years is hardly enough time to form a judgment. When you choose investments with longer track records, you can examine how they did during different kinds of market environments (bull markets, bear markets, inflationary periods, etc.). That’s much more meaningful.
• Focused – Even some seemingly simple investments can be complicated. Consider, for example, target-date mutual funds, which are popular in many 401(k) plans. Because these funds are comprised of stocks and bonds—and sometimes other asset classes—they’re difficult to judge. My advice: Stick with the simplest funds, those that hold just one asset class—a total stock market fund, for example—rather than a stew that is hard to characterize. A bonus: Simple investments usually carry the lowest costs and are most tax efficient.
• Relevant – When last I looked, this morning, Taleb and Asness were still going at it. And because investment performance is such a fuzzy topic, they could probably go on indefinitely. Fortunately, there’s a simple solution: Ultimately, the best measure of performance for you is whether you’re on track toward your own financial goals. Worry less about how your portfolio matches up with an arbitrary benchmark.