SHOP TALK: Cross-Channel Sales and Budget for Multichannel Retailers

Doing this, you employ the most reasonable best practice of all — testing! If I were you, I'd test all my direct marketing programs in this manner, and then use business rules obtained from the tests to determine how to allocate multichannel sales.

Some sales allocation methods are a bit more complex. It's really hard to determine if customers who visit an e-commerce website on Tuesday and shop in a brick-and-mortar store on Thursday placed retail orders because of the website, or if they used the website because they were going to shop in a store on Thursday. There probably won't ever be a perfect answer for these instances, regardless of what smart mathematical minds might tell us, because marketers can never truly ascertain customer intent to complete certainty.