Janet Yellen wasn’t prepared Wednesday with a full explanation of why Federal Reserve officials surprised markets so dramatically earlier this month, the kind of miscommunication that the central bank has tried to avoid and that congressional Republicans have complained about.

On March 1, market odds of the Fed raising rates at its March meeting doubled thanks to comments from Fed officials, despite the absence of any major new economic data. In the space of a week, investors went from seeing a rate hike as an outside possibility to viewing it as a lock, based only on speeches and interviews of Fed members.