The classes in the OPM schema can be viewed as an aggregation hierarchy rooted in the class Project. This reflects the fact that researchers perform experiments in the context of a project, and that in the course of performing each experiment, they might carry out some sequence of procedures and sub-procedures (including scans, plot-generation, etc.). An experiment is mainly composed by scans, but also has procedures, annotations, plots, and pictures. Procedures are used to model actions that do not involve taking real data, despite involving scans. Procedures can also contain subprocedures, which are ultimately composed by a description and eventually scans.