The increasingly popular "smart beta" ETF's are rebalanced quarterly based on some "smart" factor that is fundamentally or technically based. Most traditional "dumb beta" ETF's are capitalization-weighted. Here's the rub: In some cases, the rebalancing of these smart ETF's can change the composition and behavior of the ETF significantly. By contrast, the cap-weighted ETF's don't change too dramatically and generally maintain the same ranking of their components over time. This is important to be aware of if you use TA or any other techniques that rely on historical data. The smart beta ETF can take sharp turns in price after rebalancing and becomes a different animal than it was before rebalancing, making the price series less useful for historical studies.