In this paper we have attempted to distinguish among the good policy, good practices,
and good luck explanations of the reduction in U.S. output volatility over the last 15-20 years
using frequency domain and VAR techniques. In the frequency domain, for aggregate output
and for all of its broad demand-side and product-side components, except for durable final sales,
we cannot reject the hypothesis that the decline in variance has been evenly distributed at the
various frequencies; for durable final sales the decline in variance is concentrated at the business
cycle frequencies. Although the latter result is consistent with better inventory management,
overall, our frequency domain results lend considerable support to the good luck explanation.
Our VAR results indicate a moderately bigger role for changes in the structure of the
economy, as opposed to changes in the probability distributions of the shocks, in explaining the
decline in aggregate output volatility. However, it is still generally the case that the shocks
account for most of the decline in output volatility, a result also found by others (e.g. Simon,
2000). The results are robust to the use of monthly data, but we find somewhat weaker support21
for the good luck hypothesis when we distinguish between innovations to inventories and
innovations to final sales using the five-variable quarterly model. Overall, we conclude that,
although better practices and better monetary policies have played some role in explaining the
decline of U.S. output volatility in the past 10-15 years, good-luck is probably the leading
explanation. This suggests that, as far as output variability is concerned, it might be premature
to conclude that the reduction in volatility is a permanent feature of the U.S. economy.
Applying the same methods to consumer price inflation, we strongly reject the hypothesis
of a proportional decline in the spectrum at all frequencies, thereby ruling out the idea that lower
inflation volatility has been due to good luck alone. Our VAR results for inflation reinforce this
result; that is, changes in the structure of the economy account for the bulk of the post-1984
reduction in inflation volatility. These results support the view that monetary policy has played
a crucial role in stabilizing inflation over the past two decades.