Assessing the causal effects of financial aids to firms in Tuscany allowing for interference

We consider policy evaluations when the Stable Unit Treatment Value Assumption (SUTVA) is violated due to the presence of interference among units. We propose to explicitly model interference as a function of units’ characteristics. Our approach is applied to the evaluation of a policy implemented in Tuscany (a region in Italy) on small handicraft firms. Results show that the benefits from the policy are reduced when treated firms are subject to high levels of interference. Moreover, the average causal effect is slightly underestimated when interference is ignored. We stress the importance of considering possible interference among units when evaluating and planning policy interventions.