Looking for secondary signs of weakness

Brett Steenbarger warned about the weakening in the market back on Sunday, Feb 4, a day before $SPY dropped >6%.

The psychological takeaway is that we need to drill down and look beneath the market surface and approach each fresh set of data with an open mind. On the day we made a peak in SPY, we had 599 stocks make fresh three month highs and 199 register new monthly lows. Two weeks before that, we had almost 900 new three month highs against 135 lows (data from Barchart.com). The open mind respects price action and market strength, but also is alert to cracks beneath the surface

via Lessons in Trading and Psychology – 1: Regime Changes at TraderFeed