Federal Reserve Chair Janet Yellen next week has to decide not only whether to raise interest rates for the first time in a decade, but also how to assure markets on the likely path of future rate hikes. The central bank has held short-term borrowing costs near zero for seven years and the last thing policymakers want is for their first rate hike to trigger expectations for future increases that could knock economic growth off track. Traders currently expect the Fed to raise interest rates two or three times next year.