Bad model predictions and incredible model assumptions invite vitriol upon economists.
Luckily, measure critique can lead to better economic models. The demise of large Keynesian simultaneous-equation models ultimately yielded a vibrant literature producing VARs, DSGEs, dynamic factor models, and more. Models ought to always be improving. And many see that machine learning (ML) has the potential to keep this process going in the coming decades.
My enthusiasm for the Macro ML revolution was reinvigorated by Phillipe Goulet Coloumbe’s (PGC) inspiring work on the Macro Random Forest (MRF) and now the Hemisphere Neural Network (HNN).
The new HNN paper strikes with an informative example at the heart of economic theory: the Phillips Curve (PC).
Walking through the Covid era recursively (to ensure the model does not overfit the Covid data ex-post), PGC finds, among other things, that today’s output gap is not near zero, as existing approaches imply. He also finds that the PC coefficient of the output gap has increased reliably since 1990, contradicting the popular narrative that the PC is dead.
More generally, HNN is a new way to estimate latent economic variables that we don’t actually observe (e.g., the output gap, neutral interest rates, term premia). HNN improves upon existing methods in a number of ways.
- It dispenses with restrictive law of motion assumptions on model parameters and latent states. State-space approaches often impose that factors or coefficients evolve according to a random walk or an arbitrary AR process.
- It produces a linear output layer. So, despite all the non-linear ML, there is a final, fully interpretable end-product.
- Important predictors of the latent states can be identified, allowing for more interpretability over and above both traditional machine learning and factor-based econometric methods.
- It allows for a novel sense of volatility that addresses known weaknesses in the widely-used GARCH and SV approaches.
- As an extension of 4, the model can predict its own demise. It will explicitly tell you in real-time if its predictions are uncertain.
I believe PGC’s recent work is an important step forward in the ML-meets-macroeconometrics literature. MRF and HNN directly, intuitively, and effectively address many concerns about macro models head-on using clever modifications of widely-known ML approaches. I am looking forward to future developments. And so should you.