> **Note**: Class types are used to describe values that are instances of a class---these are the most commonly used types. To instead learn about types for class objects themselves, see [T.class_of](/docs/class-of).| sorbet.org
There are two primary metrics that we recommend to track adoption of Sorbet in a codebase:| sorbet.org
## Why does Sorbet think this is `nil`? I just checked that it's not!| sorbet.org
Recall from [Enabling runtime checks](/docs/runtime) that `sorbet-runtime` raises| sorbet.org
Sorbet is a gradual type checker, which is a blessing and a curse.| sorbet.org
This doc will cover how to enable and disable the **static checks** that Sorbet| sorbet.org
| sorbet.org
## Step 1: Install dependencies| sorbet.org
Sorbet provides the most value when it has a wealth of programmer-supplied| sorbet.org
As we've mentioned before, Sorbet is a [gradual](/docs/gradual) system: it can be| sorbet.org
RBI files are "Ruby Interface" files. Sorbet uses RBI files to learn about| sorbet.org