The December issue of Alliance will have a special feature on measuring impact, which will look critically at the range of approaches that are being taken. So how do they do it at the Bill & Melinda Gates Foundation, the world’s largest private foundation, which will soon be spending as much as $3 billion a year? In the run-up to publication of the Alliance special feature, Alliance talked to Fay Twersky, recently appointed to head up the Foundation’s new Impact Planning and Improvement Unit.