Number 11/Networks & contagion/Aggregation
The Composition Share
The quantity
The fraction of an aggregate change attributable to shifts in the mix of units rather than to change within units.
01Why it matters
It decides whether an aggregate trend describes what is happening to anyone or only to the average, and the two support opposite policies.
02Expected value
Above one half for many productivity, health and demographic aggregates.
03How it would be measured
Decompose the aggregate into within-unit and between-unit components using panel data.
04What would refute it
A share near zero would mean aggregates describe typical units faithfully.