And yet, the metric is fundamentally wrong.
At Nordstrom, we analyzed our multichannel shoppers to death. We knew the following happened, on a monthly basis --- called the "3-2-1" rule.
- The multichannel shopper visited our website three times a month.
- The multichannel shopper visited at least one store two times a month.
- The multichannel shopper purchased one time a month, usually in a store.
While still flawed, we evaluated a "Monthly Conversion Rate".
- Take all customers who visited your website in the past month.
- Of those visitors, what percentage purchased something, in any channel?
- Divide monthly purchasers by unique monthly visitors.
What is important is the business intelligence you gain from a monthly conversion rate. All of a sudden, you realize that your conversion rate isn't a measly 4.398541842304%. Instead, your website facilitates a monthly conversion rate of 37% across all channels. Heck, you're not the failure that the vendor community portrays you to be!!!!!!! You're actually more successful than you thought. That's not a bad thing.
So instead of beating me up over all the reasons this metric is wrong, work with your BI team, or with Coremetrics/Omniture to create this metric, and start enhancing the way you evaluate your business --- pick a different timeframe or metric --- just do something to advance your industry!