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How Chatter Topics are related to eachother

Knowledge Article Number 000221995
Description After following the steps to Enable and Configure Topics for Objects users may be wondering how Topics are marked as related to one another upon Viewing Details about a Topic.
Resolution If two topics are frequently co-assigned together on the same items, the application will automatically mark them as related and once they're flagged as related they appear in one another's Topic page's Related Topics list.

Two topics are flagged as related if there are at least 3 instances of both topics being assigned together on an object.

An object here refers to anything in which topics could be assigned for example, a FeedItem(Chatter Post) KnowledgeArticleVersion(A chatter post on a Knowledge article) or associated to an object's record directly if Topics for Objects has been enabled.
Example: The topics "iPhone" and "Apple" are both assigned to FeedItem1 and both assigned to FeedItem2 and also both assigned to KnowledgeArticleVersion3.  In this circumstance, they've been co-assigned 3 times so the iPhone topic page would show Apple in its Related Topics section and vice versa.

Pairs of topics are related by different amounts based on how many times they have been co-assigned (over 3). 

Example: If there are 5 objects that have both TopicA and TopicB assigned and there are only 4 objects that have TopicA and TopicC assigned then TopicA and TopicB will be more related than TopicA and TopicC and as a result, TopicB will show up before TopicC in TopicA's list of related topics.

Time exponentially decays the amount two topics are related.

Example: If there are 5 objects that had both TopicA and TopicB assigned a year ago but there are 4 objects that have TopicA and TopicC assigned today then TopicA and TopicC will be more related than TopicA and TopicB.
In detail the time decay aspect works like this:

Two co-assignments one year ago is worth as much as one co-assignment today. Two co-assignments two years ago is worth as much as one co-assignment one year ago. Generally speaking a co-assignment's worth decays by half each year. E.g. if there are 4 objects that had topicA and topicB assigned two years ago and 2 object that had them both assigned one year ago and 1 object that had them both assigned today the total relation amount would be 4/4 + 2/2 + 1/1 = 3 and they would be considered related (threshold 3). If y number of co-assignment happened x years ago it would be worth y/(2^x) where y is a positive integer and x is a positive number not necessarily an integer.

The one year half life for the worth of two topic co-assignments is a design consideration with the intent being to not have the time aspect affect the topic relation amount too much however, this may be subject to change as the Topics functionality sees future enhancements.

See Also:
Topics Privacy​
Related Topics​

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