Extrapolation: Dangers Of The Reckless Ruler by Wade W. Slome, CFA, CFP®, Investing Caffeine

The game of investing would be rather simple if everything moved in a straight line and economic data points could be could be connected with a level ruler. Unfortunately, the real world doesn’t operate that way – data points are actually scattered continuously. In the short-run, inflation, GDP, exchange rates, interest rates, corporate earnings, profit margins, geopolitics, natural disasters, financial crises, and an infinite number of other factors are very difficult to predict with any accurate consistency. The true way to make money is to correctly identify long-term trends and then opportunistically take advantage of the chaos by using the power of mean reversion. Let me explain.

Take for example the just-released October employment figures, which on the surface showed a blowout creation of +271,000 new jobs during the month (unemployment rate decline to 5.0%) versus the Wall Street consensus forecast of +180,000 (flat unemployment rate of 5.1%). The rise in new workers was a marked acceleration from the +137,000 additions in September and the +136,000 in August. The better-than-expected jobs numbers, the highest monthly addition since late 2014, was paraded across television broadcasts and web headlines as a blowout number, which gives the Federal Reserve and Chairwoman Janet Yellen more ammunition to raise interest rates next month at the Federal Open Market Committee meeting. Investors are now factoring in roughly a 70% probability of a +0.25% interest rate hike next month compared to an approximately 30% chance of an increase a few weeks ago.

As is often the case, speculators, traders, and the media rely heavily on their trusty ruler to connect two data points to create a trend, and then subsequently extrapolate that trend out into infinity, whether the trend is moving upwards or downwards. I went back in time to explore the media’s infatuation with limitless extrapolation in my Back to the Future series (see Part I; Part II; and Part III). More recently, weakening data in China caused traders to extrapolate that weakness into perpetuity and pushed Chinese stocks down in August by more than -20% and U.S. stocks down more than -10%, over the same timeframe.

While most of the media coverage blew the recent jobs number out of proportion (see BOOM! Big Rebound in Job Creation), some shrewd investors understand mean reversion is one of the most powerful dynamics in economics and often overrides the limited utility of extrapolation. Case in point is blogger-extraordinaire Scott Grannis (Calafia Beach Pundit) who displayed this judgment when he handicapped the October jobs data a day before the statistics were released. Here’s what Grannis said:

The BLS’s estimate of private sector employment tends to be more volatile than ADP’s, and both tend to track each other over time. That further suggests that the BLS jobs number—to be released early tomorrow—has a decent chance of beating expectations.

Now, Grannis may not have guaranteed a specific number, but comparing the volatile government BLS and private sector ADP jobs data (always released before BLS) only bolsters the supremacy of mean reversion. As you can see from the chart below, both sets of data have been highly correlated and the monthly statistics have reliably varied between a range of +100k to +300k job additions over the last six years. So, although the number came in higher than expected for October, the result is perfectly consistent with the “slowly-but-surely” growing U.S. economy.


Source: Calafia Beach Pundit

While I spend much more time picking stocks than picking the direction of economic statistics, even I will agree there is a high probability the Fed moves interest rates next month. But even if Yellen acts in December, she has been very clear that this rate hike cycle will be slower than previous periods due to the weak pace of economic expansion. I agree with Grannis, who noted, “Higher rates would be a confirmation of growth, not a threat to growth.” Whatever happens next month, do yourself a favor and keep the urge of extrapolation at bay by keeping your pencil and ruler in your drawer.


Wade W. Slome, CFA, CFP®

Plan. Invest. Prosper. 

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