Manipulating index numbers

I have a producer price index which is the composite of 9 various commodity prices including that of coal. I want to remove the effects of coal price changes from the original index. I have data on coal prices. Does anyone know of a method that I could use to filter out the effects of coal price changes from the original index?

• $P_{agg}-P_{coal}Q_{coal}$? – Herr K. Jul 5 at 18:57
• Do you happen to know how the index is calculated (Paasche, Laspeyres, etc.), and the components (price changes, weights) for the individual components? Whether a particular method will work for your purposes depends on what information you have available to you. – dismalscience Jul 5 at 21:41
• @dismalscience I don't know the method, but what if the index is an unweighted average of the commodity prices? – london Jul 6 at 20:14

Index = (1 + Product1ChangeIn%)^(1/9) * (1 + Product2ChangeIn%)^(1/9) ...

Index / (1 + CoalChangeIn%)^(1/9)