Data Mining vs Out of Sample Data

So in this last post, I data-mined the hell out of the S&P500 index (well ok SPY) and found an “anomaly”: every time SPY drops more than 1% from the previous close to the current close, you wait (that’s Day 0). You then buy at the close 13 days later, and sell at the close of Day 14. This showed significantly better return than if you did the same thing but owned all the Day 16s instead. Here’s the graph from the last post.

Screen Shot 2016-02-04 at 8.21.02 PM

But with only 177 samples of data between 2010-2015, that’s probably just a fluke….right?

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