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The ODM model assumes that a study's clinical data will consist of several kinds of entities. These include subjects, study events, forms, item groups, items, and annotations.

An item is an individual clinical data item, such as a single systolic blood pressure reading. Items are collected together into item groups.

An item group is a closely related set of items that are generally analyzed together. (Item groups are sometimes referred to as "records" and are associated with "panels" or "tables".) Item groups are aggregated into forms.

A form is analogous to a page in a paper CRF book or electronic CRF screen. A form generally collects a set of logically and temporally related information. A series of forms is collected as part of a study event.

A study event is a reusable package of forms usually corresponding to a study data-collection event.

A study protocol design may include zero or more planned subject visits at which data will be collected. Each planned visit may correspond to one or more ODM study events and ODM study events may be used for more than one subject visit.

A subject is a patient participating in the study.

An annotation is a comment applied to a subject, study event, form, item group, or item. Annotations can also be applied to pairs of entities.

The metadata of a study describes the types of study events, forms, item groups, and items that are allowed in the study. The metadata entities include StudyEventDefs, FormDefs, ItemGroupDefs, and ItemDefs.

A StudyEventDef describes a particular type of study event (mostly by listing the types of forms it can contain).

A FormDef describes a particular type of form.

An ItemGroupDef describes a particular type of item group.

An ItemDef describes a particular type of item.

The clinical data of a study will typically have many actual study events corresponding to each StudyEventDef, many actual forms corresponding to each FormDef, and so on.

An ODM file (like any XML file) consists of a tree of elements. The clinical data elements in an ODM file represent either the state of a clinical entity or a change to the state of that entity. These elements include the SubjectData, StudyEventData, FormData, ItemGroupData, ItemData, and Annotation elements.

Each such element contains key attributes that identify the data entity that it provides information for. Frequently, many data elements will correspond to a single data entity. This can occur when a series of updates are being applied to a single entity, or when an audit trail is being represented.

Similarly, there are XML elements corresponding to metadata entities.

Note: Not all clinical studies organize their data into visits or forms. For such a study, it is appropriate to have a single common study event per subject, and/or a single common form per study event.