I was member of a project commissioned by the Catalan Tax Agency to calculate the "Tax Gap" of regional wealth taxes.

A summary of the methodology and the results can be found at:

Durán-Cabré, J.M., Esteller-Moré, A., Mas-Montserrat, M., Salvadori, L. (2019): "The tax gap as a public management instrument: application to wealth taxes", Applied Economic Analysis (previously so-called REA), forthcoming.