This is how the pet score is calculated here and why they decided to use this rating system.
Basiclly 1 unique poor quality pet would give you 2 total points where as the same pet but rare quality gives you 5 total points. Plus points for overall average pet level.
In your example the person with 500 poor pets would have a score of 1000 +(avg pet level x4) and the person with 430 rare pets would have a score of 2150 +(avg pet level x4).Original link
How is pet score calculated?
Your score is primarily determined by the total number of unique pets you've collected and the quality of those pets. To a lesser extent, your pets' levels play a role. Here is the formula we use to calculate pet score:
2*(total unique pets)
+ 4*(avg. pet level)
+ 1*(total common pets)
+ 2*(total uncommon pets)
+ 3*(total rare pets)
+ 5*(total epic pets)
+ 10*(total legendary pets)
Pet ScoreOnly unique pets are factored into pet score. Also notice that even though we use a multiplier of 4 to determine the level portion of your score, it's your average pet level that's being assessed. So you'd have to level a lot of pets by several levels to see a real difference in your score.
How do you weigh duplicate pets into my stats and pet score?
We don't. Duplicate pets do not count toward your stats or pet score in any way.
For example, if you have 75 pets at the max level, but 10 are duplicates, your max level stat will be 65 (despite any pet leveling achievements you may have earned). Any pet that's tinted in orange is considered a duplicate. Only unique pets are used to determine your pet score and other statistics.
How do you determine which pets are considered unique or duplicates?
The highest level of the highest quality pet (among duplicates) is considered your "unique" pet. Unique pets are the ones factored into your pet stats and pet score. We prioritze pets by quality first, followed by level, and then by whether or not the pet was given a custom name:
Quality → Level → Name
For example, say you have three of the same pet: a level 23 Uncommom, a level 18 Rare and a level 21 Common named Bob. The level 18 Rare would be considered your "unique" pet because it is the one with the highest rarity (even though it also has the lowest level).
How can someone with less pets or pets of a much lower level have a higher score than me?
If you look at the formula, you'll notice that the quality of your pets plays a significant role in determining your score. So if someone has more rare pets, their score might be higher even if their unique pet count and average pet level are lower than yours. In fact, even if you have more rare pets than they do, their score might still be higher due to having several more uncommon pets while you have more common and poor quality pets.
Remember — pet score was designed to reward all facets of pet collecting, from rare-hunting to pet-leveling. So the sheer number of pets you have isn't always going to put you on top
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