How Long Will ‘Transitory’ Inflation Last?

 | May 13, 2021 08:24AM ET

The US Consumer Price Index surged in April, topping estimates by a wide margin. The question is whether this is the start of an inflationary wave or just the latest run of noise?

Reliably answering that question will take several months at the least, but if we ignore that caveat the numbers for last month certainly look worrisome. Let’s start with core CPI, which strips out the food and energy noise, a filter that helps identify the underlying trend. On this basis, the monthly surge in core CPI is an extreme outlier: the 0.9% increase in April is the highest since inflation was raging 40 years ago. That could be a sign that the jig is up and pricing pressure is heating up to the point that the old paradigm no longer applies. But outliers can also be one-time shocks that are mostly light and heat. Outliers, by definition, after all, aren’t usually indicative of the trend. Unfortunately, we don’t yet know which applies and so caution is still recommended for reading too much in one number.