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?
The best way is taking the index "ingredients" and rebuild it... assuming you manage the index data on a DB or on an excel file, that should not be too difficult
Otherwise, assuming you're index ingredients are averaged equally, then in a sense, the index is calculated as:
Index = (1 + Product1ChangeIn%)^(1/9) * (1 + Product2ChangeIn%)^(1/9) ...
So based on it, without coal, you're index should be:
Index / (1 + CoalChangeIn%)^(1/9)