It's hard to know the origin of exported goods. Let's say your country exports wood, and you have imported screws. If you export furniture, do you assume it is

It's hard to know the origin of exported goods. Let's say your country exports wood, and you have imported screws. If you export furniture, do you assume it is built using screws, and measure the country's added value by subtracting the value of the screws from the value of the furniture? Or maybe you believe it is glued, but actually uses screws - now you have overvalued. And what if you import another, more expensive kind of screw from Japan, which looks identical but has different properties? Do you know which ones are being exported, and therefore what value the country is actually adding? And to think this is only a simple example!