Simple (Unweighted) Aggregative Method

Once you need to track a group of items together — say a full basket like housing, food, medical care and transport — rather than one commodity at a time, the simple aggregative method builds a single combined index for the whole group. The idea is to express the aggregate (sum) of the group's prices in the current period as a percentage of their aggregate in the base period:

Here $\sum p_0$ is the sum of the unit prices of all items in the base period, and $\sum p_n$ is the sum of their unit prices in the current period $n$. The same idea, applied to quantities instead of prices, gives the quantity index: