Comment by tomrod

Basket goods, basically.

Price of good i x Quantity of good i. Quantity is fixed year to year. So a loaf of bread, a gallon of milk, a TV, etc.

Sum those up across a reasonably representative basket, then compare that sum to the same quantity and new prices in a future year.

sum(P_i_new year x Q_i) / sum(P_i base year x Q_i) - 1 --> change in CPI

Hamburgers might be more expensive, but TVs, toilet paper, and dog kibble might not be.