People often think that if someone is good at one thing, he's probably good at many similar things.
Andrew Carnegie seldom thought that way. When he hired people, he did so based on the idea that they were specialists. He insisted that his employees stick to their specialties. He did not demand versatility from anyone. He did not go too far in making generalizations about his employees.
The public tells us that general skills are more common than specific skills. It places spotlights on people's versatility, and seldom presents the idea that someone can be well suited for one thing and not at all suited for something very similar.
Jay Leno had a successful career doing stand up--and then he moved on to hosting a late night variety show. Steve Harvey started off with stand up, and then he branched off to hosting talk shows and game shows. These two stand up comedians showed that they were good at one thing, and then they showed they were good at some similar things.
Society spotlights those examples, and acts like they tell you what you can come to expect from successful stand up comedians. Society suggests that the typical successful stand up comedian will perform at a high level when it comes to writing and delivering late night monologue jokes, starring in skits, interviewing people, and hosting game shows.
But if you study stand up comedians in general, you will find that the overwhelming majority of the successful ones will not succeed when moving on to another form of comedy. It's far, far, far more common for them to produce subpar results when doing another type of comedy. Almost ever single successfu stand up comedians in the world has failed repeatedly when it comes to other comedy related endeavors. For instance, the typical successful stand up has starred in three failed TV pilots (after developing a successful stand up career).
Here's an even better example. The typical writer who creates a successful sitcom will go on to create one failed sitcom after another. His successful show will be followed by three to seven shows that are far from successful. Part of this is because the writer's abilities lie in creating a very, very specific kind of sitcom, and he lacks the versatility to properly develop a slightly different kind of sitcom.
The public focuses way more on the Leno and Harvey examples. It makes you think that versatility is common--which in fact, it's rare.
Andrew Carnegie understood the rarity of versatility, and ran his organization accordingly.