Because experience doesn’t necessarily make people better at what they do:
Extensive research in a wide range of fields shows that many people not only fail to become outstandingly good at what they do, no matter how many years they spend doing it, they frequently don’t even get any better than they were when they started. Auditors with years of experience were no better at detecting corporate fraud—a fairly important skill for an auditor—than were freshly trained rookies. When it comes to judging personality disorders, which is one of the things we count on clinical psychologists to do, length of clinical experience told nothing about skill—“the correlations,” concluded some of the leading researchers, “are roughly zero.” Surgeons were no better at predicting hospital stays after surgery than residents were. In field after field, when it came to centrally important skills—stockbrokers recommending stocks, parole officers predicting recidivism, college admissions officials judging applicants—people with lots of experience were no better at their jobs than those with very little experience.
This doesn’t mean you can’t learn from experience. It means that learning from experience is not automatic. Some people can make the same mistakes over and over again and be none the wiser for it.
This is why I don’t classify engineers by experience: mobile vs. web, iPhone vs. Android, Ruby vs. Python. I look for intelligence, drive, fundamental skills, and an ability to get stuff done. Six months into the job, a great engineer who started with no experience on a platform will outperform a mediocre engineer who started with several years' experience.
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