Professor Paul Black: Formative and summative assessment pedagogy: The teacher's role. (Full version). 

http://www.educationscotland.gov.uk/video/p/paulblacktheteachersrole.asp

Thank you Education Scotland

Broken down video http://www.educationscotland.gov.uk/resources/p/paulblack.asp?strReferringChannel=video&strReferringPageID=tcm:4-823014-64&class=l2+d134742