Reader Arthurian writes in the wake of the 3% growth rate (SAAR) reported for 2017Q3:
Back in January Menzie Chinn said he thought growth in the 3.5-4% range was “unlikely”. I wonder if he is having second thoughts now.
I’m going to show two pictures deploying at most undergraduate statistics to show why I — like most numerically literate people — still think sustained 3.5% growth is unlikely.
First, consider what a naive statistical model — an ARIMA(1,1,1) estimated over the 1986-2017Q3 period — says, as compared to a 3.5% or 4% growth rate.
Figure 1: Reported GDP (blue), ARIMA(1,1,1) dynamic forecast (red) and 64% prediction interval (gray lines), implied path for 3.5% sustained growth (pink) and for 4% (light green), all billions Ch.2009$ SAAR. Source: BEA 2017Q3 advance release, author’s calculations.
Note that by estimating the regression starting in 1986 when the trend growth rate was relatively high, I am slanting the results toward projecting faster growth. Even then, the 3.5% growth rate is at the uppermost edge of the 64% prediction interval. 4% growth is even more laughable.
Second, now consider what sustained 3.5% growth implies for the output gap. For that calculation, one needs an estimate of potential GDP; for want of a better measure, I use CBO’s estimate. This is shown in Figure 2.
Figure 2: Reported GDP (blue), implied path for 3.5% sustained growth (pink) and potential GDP (gray), all billions Ch.2009$ SAAR. Source: BEA 2017Q3 advance release, CBO (June 2017), author’s calculations.
Assuming sustained 3.5% growth, the output gap by end-2019 would be 4.2%, 5.9% by end-2020. Of course, CBO could be terribly misguided about the trajectory of potential GDP. But in order to believe potential GDP can grow at 3.5%, I really do believe that it can only be done by adherence to Spaceology. Refer to Figure 3, reproduced from my January post, and look at what the orange bar — labor productivity — has to do in order to hit 3.5% potential growth.
Figure 3: Contributions to annual growth in potential GDP growth, from labor force augmentation (blue bar), and from labor productivity growth (orange bar). DJT’s target of 3.5-4% shown as pink range. Source: CBO, Budget and Economic Outlook, January 24, 2017, Table 2-3; and Trump-Pence website