I disagree.
For example, the ETC CRUD attempts to track an index (DJAIGCL). This index tracks the daily % change of WTI Crude Oil future(s). Notice it's the 'change', not the absolute price. Every two months, the index switches over (rolls) to a later contract. If I have attached the graph correctly, you sould be able to see that the index follows the ups and downs of the futures contracts accurately, but not the absolute price of the contracts. The roll occurs between 6th-12th Feb.
All prices rebased to 1 at start date of 20th Jan.
In this example from 20th Jan to 19th March, the contract price being followed increases nearly 12%, whilst the index and ETC values are much lower (the ETC has actually lost nearly 5%). This difference in value is due to Contango (later contract price being higher than near contract price) and has severly affected commodity trading in recent months.
(p.s. I learnt all this the hard way :angry: )