When it comes to commodity exchange traded funds (ETFs), choosing among the many available isn’t always a simple task.

Investors have realized that prices for things like oil and corn can surge even as the rest of the market spoils. Ian Salisbury for The Wall Street Journal reports that many financial advisors are now keeping a small portion of investors’ assets in commodities to smooth overall volatility in a portfolio.

But constructing an index of commodities can be a challenge. Stock indexes often contain companies weighted by market capitalization – but that’s not possible with commodities. This means index designers have to take a different route, and it yields wildly different results.

Compare the iShares S&P GSCI Commodity Indexed Trust. It has 78% in energy and 2% in precious metals. As a result, it is up higher despite the recent declines in oil. The Greenhaven Continuous Commodity Index has 47% in agriculture and 18% in energy, and it’s up 2% since launching in late January.

Many other commodity ETFs and exchange traded notes (ETNs) sit somewhere between those two extremes.

Index designers have different ways of decided how much of which commodity to include. One way is to estimate how much of a commodity is produced in a year. But commodities are not like equities and the closest thing you can get to market-cap weighting is to estimate production.

• iShares S&P GSCI Commodity-Indexed Trust (GSG), up 11.2% year-to-date
• GreenHaven Continuous Commodity Index (GCC), down 0.3% since Jan. 24 inception
• iPath Dow Jones-AIG Commodity Index TR (DJP), up 2% year-to-date

Tags: Agriculture, Commodity ETFs, DJP, Energy, GCC, Green ETFs, GSG, Oil, Precious Metals