Efficiency

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Profession efficiency increases as you practice that profession, but your efficiency in the other professions decreases. Simply put, the more you practice the same profession, the more effective you are at it.

Efficiency is a percentage measure of how focused a player is on a single Profession. 100% efficiency is shared among Gathering Professions, and a different 100% efficiency is shared among all Crafting Professions.

Factors Affecting Efficiency

Upon increasing the efficiency of a profession, the non-zero efficiency of the least-used profession of the same type decreases by the same amount. If the efficiency of the least-used profession reaches zero during the decrease, the remaining efficiency will be taken from the next least-used profession, and so on.
Efficiency increases when Gathering from resource nodes with the same tier as the corresponding profession's Mastery Level, or when Crafting items with high enough gear score compared to the corresponding profession's Mastery Level.

Bonuses

Upon increasing a profession's mastery level, that profession will recieve various bonuses. In gathering professions, higher efficiency yields a higher chance of identifying a rarer resource, and reduces the chance of failing the collection of a resource.
In crafting professions, higher efficiency increases the chance of crafting items with higher gear score.

Upon reaching level 40, players will gain access to unique profession-specific bonuses which affect gameplay.