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

If you don't see any difference in sales across any of your channels, then you know sales from email are incremental and should be attributed to email. If paid search sales increase 10 percent when emails are sent, you know to take 10 percent of future paid search sales and attribute them to email marketing. If affiliate sales decrease 10 percent when emails are sent, you know email cannibalizes affiliate marketing. Therefore, you re-allocate 10 percent of email sales to 
affiliate marketing.