*** Overview - Describe the feature from an end-user perspective.

Show a list of all guildmembers who were part in any of the raids which are still existing in the history of raids.
The list should show how many raids are in the calculation.
Per member, a number should show how many of the raids they were there  as well as expressed in a percentage over all the raids.
Ability to sort on the percentage column to have a quick ranking overview as well as export to phpBB would be helpfull.
If I remove some raids from the history, then of course it will only show the statistics for those raids left.
By controlling which raids are kept in history it will be easy to create overviews based per week or per month depending on the usage.

Example of possible output:
====================
Totals over 5 raids:
Member  #available %
name3 ------ 5 ------- 100
name2 ------ 4 ------- 80
name1------- 2 ------- 40
name4------- 0 ------- 0
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*** Functionality - Describe the feature from a development perspective.

I think you have all needed data already available in the history of raids. You 'only' need to create some function fo gather data over raids (I hope)

*** Notes - Is there any additional information to provide regarding this feature?

*** Use cases - How would a user make use of this feature?

Case 1)
To determing ranks based on availability/presence in raids (regardless of the total time)
In our case the people who are more than 70% available for raids will get a higher rank to reward them. At this moment it is very cumbersome to find out what the individual raid attendance percentages over several raids is.

Case 2)
If export to phpBB is available to have a transparant overview posted on the forum to show everybody who is most active and who not.

Case 3)
In case people are inactive for several weeks , this list will show who should be more motivated or even kicked from a guild.