A new study shows how a useful rule of thumb can become an expensive assumption when it starts deciding which customers deserve attention. I’ve often referred to the 80/20 rule (aka the Pareto Principle) in marketing and business because it feels right. A relatively small share of customers, decisions or effort DO often account for a disproportionate share of whatever outcome. I’ve usually meant that as a broad observation, with zero expectation that somebody counted everything precisely. Now...