How much material damage did the Northmen actually do to ninth-century Europe?

de bene esse: literally, of well-being, morally acceptable but subject to future validation or exception

http://www.medievalists.net/2011/02/20/how-much-material-damage-did-the-northmen-actually-do-to-ninth-century-europe/

How much material damage did the Northmen actually do to ninth-century Europe? - Medievalists.net

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The aim of this dissertation is to examine the material damage the Northmen perpetrated in Northern Europe during the ninth-century, and the effects of their raids on the economy of the Carolingian empire.