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The Standardized Vocabularies are constructed with a few principles in mind. Not every principle can be executed to perfection, but it represents a general motivation and direction of the ongoing improvement and development process:
The top-down problem has the additional difficulty in that Classifications are not always generally agreed on. For example, whether diabetic complications are part of the Condition Diabetes or are separate is differently answered by the respective SNOMED and ICD-9 hierarchies.