http://www.bloomberg.com/news/articles/2015-07-22/here-s-what-s-really-driving-the-commodity-index-lower
Clearly it hasnt been commodities week, month or year. But focusing on the worst-performers doesnt tell the full story.

The Bloomberg Commodity Index fell 7.2 percent this month, touching a 13-year low on July 22.
The biggest losers are crops such as coffee and sugar, each down 27 percent. Theyve fallen because of a weak currency in the most important producer, Brazil, and abundant inventories after years of surpluses, according to Commerzbank. (Theyre also very weather-driven.)

But those arent actually the reasons driving down the whole index. The biggest contributor to the indexs decline has been industrial metals such as copper and nickel, down 12 percent and 23 percent this year, respectively. Theyre falling because of concerns that the tumbling Chinese stock market betrays sapping demand from the worlds biggest consumer.

Whats behind the discrepancy? More valuable commodities like oil and gold (down 12 percent and 6.8 percent) have heavier weightings in the index (and most exchange-traded products and other portfolios). As it happens, theres a lot of overlap between the smallest members and the biggest nominal declines.