Why doesn't CPI include commodities?

Krugam answers:

"The problem of embedded inflation applies only to prices that are set at fairly long intervals — especially to wages, which are usually set only once a year. There’s no comparable problem with commodities like wheat or oil, where the price changes minute by minute, and goes down as easily as it goes up. It may sound perverse, but embedded, hard-to-reverse inflation is only a problem for parts of the economy with relatively sticky prices.

So to get a sense of whether embedded inflation is becoming a problem, you have to purge the highly volatile prices — basically, commodities — from the picture. That’s why the Fed focuses on “core” inflation that excludes food and energy: it’s not a nefarious scheme to ignore the real hardships people face, it’s an attempt to figure out if inflation is getting built into the system."