Quality.
A lot of the (official) emphasis on formative assessment in the classroom (or AfL - assessment for learning, they call it in school-land) is down to this guy, Dylan Wiliam, from when he was an adviser to the government:
http://www.journeytoexcellence.org.uk/videos/expertspeakers/formativeassessmentdylanwiliam.asp
There was also a TV series - 'The Classroom Experiment' - tracing his tinkering with the teaching practices of a regular secondary:
http://www.bbc.co.uk/programmes/b00txzwp
Essential watching, it was! There are still some clips on there...