A placard hangs in Professor Clayton Christensen's cluttered office at Harvard Business School with the handwritten message "Anomalies wanted" on both sides - one facing in, the other out, so passers-by can see it.
It has taken on added meaning since 2014, when a fellow Harvard academic, historian Jill Lepore, wrote a scorching critique of his best-known theory - disruptive innovation - in the New Yorker magazine. Prof Christensen, whose genial charm is legendary among his students, was wounded.
In an interview about his new book Competing Against Luck, he still sounds hurt. He says he asked the Harvard authorities what he could do about her article, which he still categorises as scholarly dishonesty (they advised him not to pour fuel on the fire). He even waited outside Prof Lepore's office hoping she would come out and he could discuss her approach (he says she told him it was "none of his business", which she denies).
The double-sided message reminds him and his supporters of what he learnt from the episode.
"It helped me realise that, Clay, you don't have to be worried if you didn't get something right because it just gives you an opportunity to improve the idea," he says in the interview. "And boy, that's been a useful insight."
Disruptive innovation describes new products that come from the lower end of a market that incumbents have neglected. One example in Prof Christensen's 1997 best-seller The Innovator's Dilemma was the way mini-mills challenged traditional steel companies.
COMPETING WITH LUCK: THE STORY OF INNOVATION AND CUSTOMER CHOICE By Clayton Christensen, Taddy Hall, Karen Dillon and David Duncan (Harper Business)
Prof Lepore in her article claimed it was a "questionable" theory and pointed out that some of the incumbents he mentioned never succumbed to innovative challengers, while some of the disrupters had in fact disappeared.
Competing Against Luck is not intended as an answer. In it, Prof Christensen expands on his parallel "jobs to be done" theory, which he has been teaching, and selling - via consulting firms he founded - for almost as long as disruptive innovation. But the book reveals interesting links between the two ideas, and with the 2014 controversy.
Describing the genesis of "jobs to be done", Prof Christensen writes of a meeting in the 1990s when he realised that "for years, I'd been focusing on understanding why great companies fail, but I realised I had never really thought about the reverse problem: How do successful companies know how to grow?"
Jobs theory aims to explain why customers "hire" particular products and services. The classic illustration is the milkshake dilemma: A fast-food chain wanted to know how to sell more milkshakes and spent months asking customers how it should improve the product, to no avail. Only when the company started asking why customers bought the drink did they work out that, in the morning, commuters purchased it as the ideal accompaniment for a long and boring drive to work. In the afternoon, though, it did a different job, for parents treating their children.
By identifying the different jobs for which customers "hired" the shake - and the different products against which it competed - the chain gained a better idea of how to develop it. Rather than making a series of hit-and-miss bets on innovation, it was able to "compete against luck" and introduce new products that were more likely to hit the target market.
In spite of the folksy and accessible style of Prof Christensen and his three co-authors, "jobs to be done" remains a slightly awkward concept. That may explain why it has not yet gained the high profile of the more easily absorbed disruptive innovation. Prof Christensen says "you need to have the life of Methuselah in order to really have any impact". But he maintains that the theory could prove more enduring than disruptive innovation.
One reason jobs theory may flourish is that the towering Prof Christensen - 2.03m in his stockinged size-16 feet - remains a charismatic frontman for his ideas, despite various health crises including diabetes, heart problems, cancer and a stroke that still occasionally leaves the 64-year-old struggling to recall precise terms. Another reason is that asking about "jobs to be done" is a way for firms to avoid falling prey to the disrupters he described in The Innovator's Dilemma.
The latest book quotes Harvard marketing professor Theodore Levitt's insight that "people don't want to buy a quarter-inch drill. They want a quarter-inch hole".
It points out, though, that managers quickly lose sight of the context in which customers buy what they produce. The job to be done is "the organising unit in a start-up", the book says. But as companies grow, unnecessary bureaucracy builds and managers define themselves by the processes they oversee. A proliferation of noisy information distracts them, amplified by new analytical tools that "embed all kinds of false rigour" into the process.
"When you launch a product and it becomes successful, all of a sudden the data that swirls around you is about products and competitors and customers and brands and demands," says Prof Christensen. "All of a sudden, they frame their world in terms of stuff that data exists about." Instead of "relentlessly asking why", he writes, managers succumb to "the crisp precision of a spreadsheet".
At the opposite extreme from the milkshake dilemma is the one facing Apple as it continues to develop a product for customers trying to do myriad jobs - from film-making to making friends. The developers of OnStar, General Motors' in-car "service assistant", had a similarly complex task. At first, Prof Christensen writes, GM provided a "Chinese menu of many random cool things" that OnStar could do, until it settled on the idea that the service's main job was to provide drivers with peace of mind.
"The beauty of jobs theory," he says in an e-mail exchange expanding on Apple's challenge, "is that it makes complex situations far easier to understand and thus to solve."
Easier, but not necessarily easy. One reason disruptive innovation took off was that it sounded simple. In The Innovator's Dilemma, Prof Christensen described "the four laws" of disruptive technology, but some fellow academics point out the theory is only one explanation of change - and far from being an immutable law.
The way the professor absorbs attacks, incorporating them into the original theory, frustrates his critics. In a Harvard Business Review article last year defending the disruptive innovation ideas, Prof Christensen said they had been "widely misunderstood (and) frequently misapplied", while essential refinements had been ignored.
Competing Against Luck's final chapter reads like an insurance policy against the same thing happening to jobs theory. It includes a section - When the theory is 'wrong' - that invites readers to send Prof Christensen and his team anomalous cases. The chapter may as well be dedicated to Prof Lepore.