Let's say a travel company sends out a series of email messages with a variety of offers. Campaigns 1 and 3 produce average clickthrough rates of just over 4 percent; campaigns 2 and 4, however, generate rates of 6.68 percent and 6.4 percent, respectively.

"Clearly, emails 2 and 4 resonated best with our audience. We should consider modeling future emails on those two success stories," says Mark Brownlow in a post at Email Marketing Reports. "Or should we?"

There's a reason for his wariness—without segmentation you might not notice that an enthusiastic minority has masked the indifference of an uninterested majority.

"What if we had two types of people on our list," explains Brownlow. "All 100,000 subscribers are interested in travel offers, but 20,000 subscribers are cruise fanatics (Group A) and 80,000 are not really into cruises at all (Group B)."

When we consider each segment's response to the "successful" campaigns, we discover something of critical importance. Clickthrough rates for email 2 are virtually identical—Group A at 6.6 percent and Group B at 6.7 percent. But the breakdown for email 4 (a cruise offer) tells a very different story. Group A went wild with a 22 percent clickthrough rate; Group B yawned with a substandard 2.5 percent.

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