Understanding whether and how connections between agents (networks) such as declared friendships in classrooms, transactions between firms, and extended family connections, influence their socio-economic outcomes has been a growing area of research within economics.

Causal effects of a policy change on the hazard rates of a duration outcome variable are not identified from a comparison of spells before and after the policy change when there is unobserved heterogeneity in the effects and no model structure is imposed.