This is what a recent study has shown. It can even go to 3 purchases if the price is included.
The study analyzed during 3 months credit card transactions of 1,1 milion people in 10 000 stores. What the study outline, is that our purchasing behavior is so personnal that it defines our own person. This show well the potential of the big data. Indeed, no need to have complex system to have reliable data, but the abondance of it is a real opportunity to better understand people.