Sorbet has multiple Strictness Levels[1]. The two most relevant ones are `typed: false` and `typed: true`. `typed: false` is the default level and at this level only errors related to constants are reported, like this one:

https://sorbet.run/talks/RubyKaigi2019/#/14

But we'd like to catch more than just errors related to constants, like those related to missing methods, or calling a method with the wrong arguments. Errors like these are only reported in files marked `typed: true`:

https://sorbet.run/talks/RubyKaigi2019/#/19

Sorbet doesn't need to have method signatures to know whether a method exists at all, or what arity that method has.

But more than that, Sorbet ships with type definitions for the standard library. So you don't even need to start annotating your methods to type check the body of your methods, because most of the body of your methods are calling standard library things (arrays, hashes, map, each, etc.).

The statistics in those slides are sharing "out of the box, what's the highest strictness level that could be added to a file without introducing errors?" So ideally an entire project be made `typed: true`, but Sorbet can be adopted gradually, so a project can exist in a partial state of typedness. We wanted to see how painful it would be to adopt Sorbet in a handful of large, open source rails projects, and it turned out to be not that bad.

[1]: https://sorbet.org/docs/static#file-level-granularity-strict...