Erie questions test whether a federal court sitting in diversity should use state law or federal law. The safest approach is to identify the source of the competing federal rule before jumping to broad labels like substantive or procedural.
Practice in Testify →First ask whether a federal statute or Federal Rule directly governs the issue. If a valid federal rule applies and conflicts with state law, that usually resolves the choice. If no controlling federal authority exists, analyze whether ignoring the state rule would materially affect the result or encourage forum shopping or unequal administration.
Students often apply the vague 'substance versus procedure' label too early. Another trap is assuming every Federal Rule automatically conflicts with state law; read the scope of the federal rule first. Erie also does not create federal subject-matter jurisdiction—the court must already have a proper basis to hear the case.
For every problem, write two lines: 'State rule:' and 'Possible federal source:'. Identifying the federal source forces you to choose the correct branch of the analysis before evaluating policy considerations.
In Civil Procedure courses, Erie often appears late in a fact pattern after diversity jurisdiction has already been established. Keep the questions separate: first can the federal court hear the case, then what law governs the disputed issue.
In bar review, practice Erie with venue, choice-of-law, pleading, and trial-procedure questions so the trigger becomes recognizable even when the question does not announce the doctrine by name.