Neil Rackham, author of bestselling sales book SPIN Selling, observes that transformative breakthroughs in sales often come after long periods of slow progress. An early example was the decision to separate the tasks of selling a product and collecting the money from customers, which enabled more efficient sales. Another pivotal moment was the 1925 publication of E. K. Strong’s The Psychology of Selling; by offering advice on how sales representatives could perfect their pitches, the book introduced the idea that sales techniques were learnable. Rackham then notes his own involvement in 1970s research on the differences between small and large sales, which he describes as the last major breakthrough in the field.

One area that has seen major breakthroughs since the 1970s, Rackham says, is purchasing. This revolution demands a matching revolution in sales, and Rackham believes The Challenger Sale may hold the key. By the time he learned of the research that underpins the book, Rackham had grown cynical about claims that sales strategies were data-based. He was especially wary of the claim that sales representatives consistently fall into one of five personality categories, believing that the distinctions might be arbitrary. When he looked into the data, he was therefore heartened that the researchers treated these categories simply as loose clusters.