MAC, the Silicon Valley Mathematics Assessment Collaborative, grew out of the California Math Renaissance, which focused on reform through good curriculum. There the:

• workshops went well but there was not much sign of change in classrooms ("Wouldn't work with my students".....)
• curriculum replacement units were each too big a bite for teachers to swallow
• coaching was so expensive

In 1996 formative classroom assessments with Balanced Assessment and New Standards tasks became a focus of professional development, working with various districts in Silicon Valley.
Each task provides a common experience for teachers to work and discuss, and a common universe of discourse for examining student work from their own and other classrooms.