I'm wondering about the correlation between weakness in the Nikkei the day after a flat or negative close in our markets. I don't know what hours Nikkei futures trade but it seems like they are far more often than not, down a bit unless the US market gains on the day. There also seems to be a correlation on very positive days for the US market. This is just a casual observation based on watching Bloomberg TV or CNBC in the morning and is unlikely to hold up over a 20 year backtest. Since the market is constantly changing, I fail to see the value of excessively long backtesting anyway.