I may be being really naive, but why is it always an estimation when talking about how many albums an artist has sold? Isn't it just as easy as doing a tick when one is sold?

And where do you get the figures from - I use wikipedia, but obviously that has its flaws.

Also, when an artist is said to have sold X amount of records, is that just albums?

Why is the difference between shipment sales and actual sales?

Well shipment sales is how much is sold to the shops, actual sales is how much is sold by the shops to the people.