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All data comes from Nightmare League players who are in one of the world's top guilds. This is because the in-game Guilds screen provides the greatest dataset available for top players. Each dataset includes more than 200 players.
Read the question directly above. The metagame for the Nightmare League is different from that of lower-level areas. It could be described as an 'exaggerated' version of the meta, where popular items are used even more, and less-popular items are used even less. For this reason, Nightmare League data can actually be better for providing insight on the strength of different items.
Because this is the only consistent set of player profiles that can be accessed through Battle Bay. All data collection occurs within an instance of the app, so it is limited to what the app allows.
The percentage is calculated through simply dividing the total number of occurrences of an item within the dataset by the total number of players in the dataset. This means that, if every player equips two Small Shields, then the use rate for Small Shields will be 200%.
Popularity data, while potentially very insightful, is not necessarily indicative of which items are "overpowered", and of which ones aren't. Ultimately, a player's success with a loadout depends far more on their skill level than on their specific choices for the items within it.
There is no monetization occurring. I am personally covering the expenses of maintaining this website, and will continue to do so for the forseeable future.
I won't disclose the exact methodology. It is 90% automated - I have to explicitly initialize the process daily. If, someday, I am unable to do this for whatever reason, then the stats will become outdated until I can fix the issue. Luckily I don't forsee this becoming a major problem.