Chaos is proving tough to shake in an American equity market where concerns that economic growth is slowing in China is alternating with dread about the first Federal Reserve interest rate increase in nine years. “People are digging in for the market gyrating and volatility to hang around,” said one chief market analyst. The S&P 500 lost 3.4 percent in the week, the second-biggest retreat since December. Almost 550 points were sheared off the Dow average, its second-worst point drop of the year. The Nasdaq dropped 3 percent, erasing its gain for 2015, while small-caps in the Russell 2000 fell 2.3 percent. The release of data on US employment Friday ignited a 272-point swoon in the Dow. The report is the last major data point before the Fed meets on Sept. 16-17 to discuss interest rate policy.