There are a couple of potentially important changes to the CPI that will take effect in the next few months. It is worth thinking about how these will affect the data.

This matters, because the ACA (nee Obamacare) caused a large shift in where payments were coming from, and one effect of that shift was to obfuscate actual inflation in medical care. Because CPI only includes payments that consumers make, and not the ones that government provides (Medicare Part A, Medicaid), large changes in the coverage population and the significant change in deductibles caused Medical Care inflation to do things that really didn’t make a lot of sense. We know that total spending on health care grew sharply under Obamacare as Medicare, Medicaid, private health insurance, and out-of-pocket spending allrose, but medical care inflation as measured by CPI sharply decelerated over the last 15 months. It isn’t because health care is suddenly more affordable; it’s because large change in the way medical care is paid for was bound to cause large change in the measurement of medical care. It is likely that reweighting this index to current weights will cause better stability in this measure – but at a higher level than the recent 1.7% rate. Since Medical Care is the main thing holding down core PCE, this will likely make the optics worse over the next year (and see what I have already said about the optics).

      2. With January 2018 data, CPI for used cars and trucks will change from a three-month moving average to a single-month price change. The BLS says “This modification will result in an index that reflects price change closer to the reference period.”