Retail stocks are up. Or are they down? Depends where you look.

While the Standard & Poor’s 500 Retailing Industry Index has gained 23 percent this year, making it the best performing of 24 industries in the benchmark gauge, Inc. has overshadowed broader weakness. But investors can’t trade that group without shelling out fees to recreate it. Instead, the most-actively traded exchange-traded fund of retail stocks paints a different picture.

The SPDR S&P Retail ETF, which is based on an equal-weighted index, has tumbled almost 9.2 percent in 2015 and is on track for its worst year since 2008. What’s more, 2015 marks the first year since then that the large-cap group is up at the same time that the retail ETF is down. Amazon accounts for just 1.3 percent of the fund, while it represents one-quarter of the retail index.

The online retailer has kept the capital-weighted group afloat, accounting for 74 percent of its gains year-to-date, aided by heavyweights Netflix Inc. and Home Depot Inc. That’s made many investors give equal-weighed measures a second look to see “what’s really happening” within the industry, said Jim Stellakis, founder and director of research at Greenwich, Connecticut-based Technical Alpha Inc. “The performance has diverged for a while, but it’s widened recently and is getting worse.”

The retail ETF and large-cap group moved largely in sync between 2011 to 2014, with year-end gains that were within about 6 percentage points. Now the two gauges are at their widest performance differential since 2007, the retail ETF’s first full year of trading.

Slower economic growth is largely to blame for sluggish sales at U.S. retailers, and that has turned into lackluster stock performance, said Jonathan Golub, chief U.S. market strategist with RBC Capital Markets. He has a market weight recommendation on the broader consumer discretionary group.

Americans are paying more for rent and items such as smart- phone and Internet services, which are “areas of leakage” that take away from the amount they have to spend at retailers, Golub said. This “cannibalization,” along with a split in the group between “new economy and old economy” companies, has made for mixed results in recent months, he said.

“Some companies are poised to do quite well and some are inherently quite challenged,” Golub said. “I don’t see any of this getting better in the next year.”

After the retail ETF outperformed the S&P 500 by almost 245 percentage points from the bull market’s start through July 16, it has since underperformed by 13 points. The fund fell 0.3 percent as of 9:35 a.m. in New York, slumping to its lowest level relative to the S&P 500 since January 2012.

“There’s a bigger shift underway and more capital is coming out than going in,” Stellakis said, reflected by a “complete reversal” in performance.