Everyone is confusing this issue beyond belief!!!
Dont take the Terms of Trade as anything more than this..
Export Price Index / Import Price Index....
It is nothing else. Its got nothing to do with supply, demand, volume, revenue, quantities sold, or anything else. If its not in the above equation, then its got nothing to do with ToT!!!!
So an improvement in the terms of trade only occurs where either export prices increase relative to import prices, or if import prices fall relative to export prices.
The export price index and the import price index are calculated in the same way as the CPI. They take a basket of our common imports, and find an average price. Vice versa for the export price index.
The thing you need to all realise is that the Terms of Trade is what is known as an economic indicator. Its not something that is designed to tell the whole story. We wouldn't look at the ToT as a sole indicator of our export performance, or anything like that. All we would use it for, is an indication of our export prices.
It could very well be that we are exporting far less and importing far more even though the ToT demonstrates an improvement. Economic indicators dont paint the whole picture, they are one piece of the puzzle.
For example in Australia at the moment, we are using the ToT to explain why the commodoties boom is so powerfull. The ToT demonstrates that the prices of commodities are increasing, and some of our imports are falling in price. And in our case, our export volumes have increased. So it demonstrates where we stand. But this picture is painted by other economic indicators, not just the ToT