Comment by mitxela

This is how CPI is measured - with the substitutes. The desk CPI is the difference between the old hardwood desk and the new MDF desk, not the old hardwood desk and the new hardwood desk.

It's not a terrible philosophy as many people don't care what their desk is made of, and the cheaper materials are actually better in some aspects (MDF doesn't warp as it dries..) but some people do and it's resulted in inflation being massively undercounted, possibly by a factor of 10.