The SPDR S&P 500 ETF Trust (NYSEArca: SPY), PowerShares QQQ ETF (NasdaqGS: QQQ) and the iShares Russell 2000 ETF (NYSEArca: IWM) are three of the largest and most heavily traded US-listed ETFs.

As such, IWM, QQQ and SPY also have some of the most robust options activity among US-listed ETFs, providing market participants with some potentially compelling data.

“Specifically, the combined front-month gamma-weighted Schaeffer’s put/call open interest ratio (FM-GW SOIR) on the SPDR S&P 500 ETF Trust (SPY), PowerShares QQQ Trust (QQQ), and iShares Russell 2000 ETF (IWM) surged above 6.0 last week — the most elevated reading since 2011, according to Schaeffer’s Quantitative Analyst Chris Prybal,” reports Schaeffer’s Investment Research.

SPY is the world’s largest ETF. QQQ tracks the tech-heavy Nasdaq 100 Index and IWM is the largest small-cap ETF.

Related: 3 Best Performing Long Bond ETFs of Past Month

According to FactSet data, U.S.-listed stock funds saw $8.78 billion in outflows, which more than offset the $3.44 billion that was funneled into fixed-income safety plays. Among the worst off, U.S. large-cap funds experienced $22.1 billion in outflows, followed by Europe total market funds focusing on developed economies that had $2.8 billion in outflows.

SPY, QQQ, IWM Bullish for U.S. Stocks

The aforementioned options activity could prove to be bullish for U.S. stocks.

“The combined FM-GW SOIR 10-day moving average officially topped 5.0 on March 23, marking its first trip above this threshold since Nov. 3, 2016. Below is how the S&P 500 Index (SPX) has performed after previous such highs in this metric, looking back to 2008. Notice that there were no signals in 2013, 2014, or 2017,” according to Schaeffer’s.