Every experiment starts with a research idea that then evolves into a plan. Somewhere in that plan there is a set of decisions about the analysis. What do you plan to measure? How? What will you do if there's missing data? An analysis plan addresses these questions and provides a record of the decisions made to analyze your results. It also provides a justification for why you handled your data the way you did. This unit walks through the planning process, from what to plan and when, to how to write a plan (including revisions!). It also demonstrates how collaboration and preregistration can strengthen plans that result in accurate, transparent manuscript reporting.