Buy or fade the breakout?

Mid-week market update: The market strength this week was no surprise to me based on my seasonal analysis I published on the weekend (see Will a volatility collapse lead to a market collapse?). Last week was option expiry (OpEx) week, and OpEx weeks have historically been bullish for stocks. In particular, Rob Hanna at Quantifiable Edges found that April OpEx week was one of the most bullish ones of the year.

However, last week saw the SPX edge down -0.1%, and my own analysis found that April post-OpEx weeks that saw market declines tended to experience strong rallies (red bars). By contrast, the market had a bearish tilt after strong April OpEx weeks (green bars).

This historical study was conducted from 1990, and the sample size of losing April OpEx weeks was relatively small (N=8). Here is the same analysis for all post-OpEx weeks. The conclusion is the same. Strong OpEx weeks were followed by market weakness, and vice versa, though the magnitude of the effect was not as strong.

Could this week's upside breakouts of the major indices be attributable to an OpEx effect? If so, could the breakout be a fake-out?

The full post can be found here.