Monday, July 18, 2016

The HAC Emperor has no Clothes: Part 2

The time-series kernel-HAC literature seems to have forgotten about pre-whitening. But most of the action is in the pre-whitening, as stressed in my earlier post. In time-series contexts, parametric allowance for good-old ARMA-GARCH disturbances (with AIC order selection, say) is likely to be all that's needed, cleaning out whatever conditional-mean and conditional-variance dynamics are operative, after which there's little/no need for anything else. (And although I say "parametric" ARMA/GARCH, it's actually fully non-parametric from a sieve perspective.)

Instead, people focus on kernel-HAC sans prewhitening, and obsess over truncation lag selection. Truncation lag selection is indeed very important when pre-whitening is forgotten, as too short a lag can lead to seriously distorted inference, as emphasized in the brilliant early work of Kiefer-Vogelsang and in important recent work by Lewis, Lazarus, Stock and Watson. But all of that becomes much less important when pre-whitening is successfully implemented.

[Of course spectra need not be rational, so ARMA is just an approximation to a more general Wold representation (and remember, GARCH(1,1) is just an ARMA(1,1) in squares). But is that really a problem? In econometrics don't we feel comfortable with ARMA approximations 99.9 percent of the time? The only econometrically-interesting process I can think of that doesn't admit a finite-ordered ARMA representation is long memory (fractional integration). But that too can be handled parametrically by introducing just one more parameter, moving from ARMA(p,q) to ARFIMA(p,d,q).]

My earlier post linked to the key early work of Den Haan and Levin, which remains unpublished. I am confident that their basic message remains intact. Indeed recent work revisits and amplifies it in important ways; see Kapetanios and Psaradakis (2016) and new work in progress by Richard Baillie to be presented at the September 2016 NBER/NSF time-series meeting at Columbia ("Is Robust Inference with OLS Sensible in Time Series Regressions?").