There are a lot of really smart people out there who do attribution work. They use complex statistical models to try and tie orders back to the advertising that drove the order.
But attribution models have one fatal flaw ... they incorrectly attribute orders that would happen organically (i.e. without advertising) back to advertising activities, causing us to spend way too much money on marketing activities.
If you agree with me that we're over-attributing orders to marketing activities ... then give this presentation a read, where I'll share with you a mail/holdout methodology for properly assigning attribution rules.