But how much is a burning issue for investors who wonder how the economy and stocks will perform once the Fed eventually eases its buying of $85 billion a month in bonds and eventually allows short-term rates to climb. 

Some believe the influence of the Fed's policy, known as quantitative easing, has been particularly important to the performance of the benchmark Standard & Poor's 500 Index .SPX. 

"People underestimate the extent to which quantitative easing has benefited the S&P," said Robbert van Batenburg, director of market strategy at brokerage Newedge USA LLC in New York. He called the effect akin to "an athlete on steroids."