which teacher exhibited the most variable year-to-year performance over this 5 year span?

Ed exhibited essentially the most variable 12 months to 12 months efficiency over this 5 12 months span.

Answer-

Most variable efficiency means various efficiency or non-consistence efficiency.

As proven within the graph,

Jill’s efficiency was fixed all through 2005-07.

Ralph’s efficiency was fixed all through 2006-07 and in addition in 2008-09.

Tina’s efficiency was fixed all through 2007-09

Sam’s efficiency was fixed all through 2008-09.

All of them has been consistence for at-least as soon as. However, in case of Ed, all through the time, his efficiency was in-consistence or variable.