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Prior to creating a custom domain, confirm that none of the existing published domains will fit the need. Once confirmed, drafting a specification upfront, using conventions in Section x2.8.x1, How to Read Domain Specifications, is recommended to ensure expectations for the custom domain are clear. Custom domains and corresponding specifications must be created based on the three general observation classes, Interventions, Events, and Findings, described in the SDTM. In most cases, the choice of observation class appropriate to a specific collection of data can be easily determined according to descriptions of these classes in the SDTM. The majority of data, which typically consists of measurements or responses to questions, usually at specific visits or time points, will fit the Findings general observation class. 

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