I have seen many traders use the spread between historical and implied volatility as a factor when entering a trade. Does this spread actually have any predictive power? If so in what context (ex. If historical higher than implied does that phenomenon continue? Does implied usually catch up to historical?) Has there been any research done on this topic? If I wanted to test this what would be some ways I could measure it's effectiveness? Lastly, I've observed many charting packages plot implied and historical in line with each other, however we don't know the historical for today until some point in the future. Is it appropriate to view them in line or does there need to be an offset applied? (ex. The 5D Hist plotted today is backwards looking which the implied plotted today is forwards looking) Thanks!