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-# 3.8 CONCEPT_ANCESTOR
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+# 3.8 CONCEPT_ANCESTOR
+
+The CONCEPT_ANCESTOR table is designed to simplify observational analysis by providing the complete hierarchical relationships between Concepts. Only direct parent-child relationships between Concepts are stored in the CONCEPT_RELATIONSHIP table. To determine higher level ancestry connections, all individual direct relationships would have to be navigated at analysis time. The  CONCEPT_ANCESTOR table includes records for all parent-child relationships, as well as grandparent-grandchild relationships and those of any other level of lineage. Using the CONCEPT_ANCESTOR table allows for querying for all descendants of a hierarchical concept. For example, drug ingredients and drug products are all descendants of a drug class ancestor.
+
+This table is entirely derived from the CONCEPT, CONCEPT_RELATIONSHIP and RELATIONSHIP tables.  
+
+Field|Required|Type|Description
+:---|:---|:---|:---
+|ancestor_concept_id|Yes|integer|A foreign key to the concept in the concept table for the higher-level concept that forms the ancestor in the relationship.|
+|descendant_concept_id|Yes|integer|A foreign key to the concept in the concept table for the lower-level concept that forms the descendant in the relationship.|
+|min_levels_of_separation|Yes|integer|The minimum separation in number of levels of hierarchy between ancestor and descendant concepts. This is an attribute that is used to simplify hierarchic analysis.|
+|max_levels_of_separation|Yes|integer|The maximum separation in number of levels of hierarchy between ancestor and descendant concepts. This is an attribute that is used to simplify hierarchic analysis.|
+
+## Conventions 
+
+  * Each concept is also recorded as an ancestor of itself.
+  * Only valid and Standard Concepts participate in the CONCEPT_ANCESTOR table. It is not possible to find ancestors or descendants of deprecated or Source Concepts.
+  * Usually, only Concepts of the same Domain are connected through records of the CONCEPT_ANCESTOR table, but there might be exceptions.