This document discusses analyzing errors in learner language, known as interlanguage. It explains that learner language develops its own systematic rules as learners learn a new language, rather than just being a collection of mistakes. The document outlines the procedure for analyzing learner errors, which involves identifying errors in recordings or writing, reconstructing what a native speaker would have said, and explaining the causes of errors, such as transfer from the native language or developing rules of the new language. The goal of this error analysis is to better understand a learner's rule system as their interlanguage develops.