Pay attention to the average number of merchandise divisions your average twelve month buyer purchases from.
Merchandise divisions represent a sign of trust. You don't necessarily buy online and in stores because of trust, you do it because of convenience.
If you shop at Nordstrom and buy Mens and Womens, you do this because of trust, you're including the whole family, for crying out loud.
Measure the average number of Merchandise Divisions a twelve month buyer purchases from.
When this number starts to decrease, you're doing something wrong, you may be headed into the Death Spiral.