There's one key economic model that the Federal Reserve has struggled to understand, Charles Evans, the president of the Federal Reserve Bank of Chicago, told CNBC on Wednesday. "We don't have a great understanding of why it's gotten to be so flat," Evans said about the so-called Phillips curve. This model states that as the unemployment rate decreases, inflation should pick up with more people in work. However, despite decreases in the U.S. unemployment rate, which currently stands at about 4 percent, the inflation rate has kept falling since the start of the year. It is the Fed's main aim to stabilize prices at ... (full story)