Two things are true about the companies that just lifted U.S. stocks to the biggest rally in four years: their fortunes are most tied to the economy, and they're the least loved by credit investors. Both are reasons for optimism.

Energy, materials and transportation shares surged an average of 12 percent in the last two weeks, beating the Standard & Poor's 500 Index by 4.8 percentage points month-to-date, the most since October 2011. The jump, which included gains in companies from Chesapeake Energy Corp. to Freeport-McMoRan Inc., followed nine months when these groups lagged behind the benchmark gauge by 14 percentage points.

The change is a sign to some investors that stock buyers are getting over their paranoia about the Federal Reserve and looking for bargains instead of simply buying the dip. It's an about-face from last month, when technology and health-care companies led a rebound from August's losses that fell apart after 15 days.

"I respect this rally much more than the one that took place in early September," said Michael Shaoul, chief executive officer of Marketfield Asset Management LLC in New York. Because of strength in credit-sensitive stocks, "this particular rally has a better chance of ending this specific period of volatility," he said.

Volatility that pushed measures of stress to the highest levels since the financial crisis happened as the U.S. economy proves resilient in the face of a global slowdown. While economists surveyed by Bloomberg this month forecast gross domestic product will expand 2.6 percent in 2016, down from 2.7 percent in a poll in August, that's still better than the 2.1 percent for the Group of 8 nations as a whole.

The S&P 500 has gained 4.9 percent since the start of October, including a 3.3 percent advance last week that was the largest of 2015. Over the span energy stocks have advanced 12 percent, materials stocks almost 11 percent and transportation shares 7.1 percent, in each case exceeding the S&P 500's 6.8 percent rally from Aug. 25 to Sept. 16.