Discussion:

Concepts that occur together in the same "entries" in some information source. The relationships represented here are obtained from machine-manipulation of the information source. Co-occurrence relationships may exist between similar concepts (e.g., "Atrial Fibrillation" and "Arrhythmia") or between very different concepts that nevertheless have some important connection in the field of biomedicine (e.g., "Atrial Fibrillation" and "Digoxin"), or between a primary concept and a qualifier e.g., "Lithotripsy" and "instrumentation". A co-occurrence relationship can exist between two concepts that have no other apparent relationship, although the frequency of such co-occurrences will be small.

In the current Metathesaurus, there are two sources of co-occurrence data: MEDLINE and AIRHEUM. From MEDLINE, co-occurrence data was computed for concepts that were designated as principal or main points in the same journal article i.e., the co-occurrence counts do not include articles in which either or both of the concepts were present and indexed in MEDLINE but not designated as main points. (A concept is considered to be a main point if the * is attached to the main heading or any of its subheadings.)

Two overall frequencies of MEDLINE co-occurrence are provided: one for MEDLINE data from 1996 through 1998 (MED) and one for MEDLINE data from 1990 through 1995 (MBD). Separate counts are provided for the frequencies with which the first concept was qualified by different MeSH qualifiers or by no qualifier at all when it co-occurred with second concept. There are separate entries for each direction of the co-occurrence relationship. The related subheading occurrence information in each entry belongs to the first concept in the entry and is therefore different for each direction of the relationship.

In addition to the specific qualifier information associated with two co-occurring concepts, this element also includes in entries with LQ and LQB values for type of co-occurrence, totals for the number of times each main concept was qualified by a specific subheading or by no subheading.

The AI/RHEUM co-occurrence data represent the co-occurrence of diseases and findings in the AI/RHEUM knowledge base, i.e., the diseases that co-occur with a particular finding and the findings that co-occur with a particular disease. Each disease/finding pair can co-occur only once in the AI/RHEUM knowledge base.