“Oil service firms are the most volatile part of the oil and gas industry. They tend to lead the way up when crude is rising, but they fall much harder than the large, integrated firms when the cycle turns down. Couple this downside exposure with the highly concentrated nature of this particular basket -- only 18 stocks in all -- and you've got a real hot potato. Handle this one with care.” – http://www.fool.com/investing/etf/2007/07/18/7-ways-to-win-with-energy.aspx