A drop is an item obtained by killing a monster. No source for the below information has been given, but this link to the French forums provides an explanation which supports these claims.
Each drop has three values associated with it: Drop Rate, Prospecting Lock, and Drop Limit.
The drop rate refers to the probability of a monster dropping an item after it is defeated in combat. An item's drop rate can be improved by the prospecting of an individual player.
Example: A 4% base drop rate would become 7% with 175 prospecting (i.e., 4 multiplied by 1.75).
Although each player in a fight rolls separately, a group has a statistically better chance of dropping the more players it has, and the higher each member's individual prospecting is.
Drop Rate Stacking
Statistically, the more people in a fight, the better the odds that at least one person in the fight will get a given drop. The probability of at least one person getting a drop is calculated as follows:
- D = Probability of drop expressed as a decimal (1% = .01)
- PPx = Each Character's prospecting (PP1=char1, PP2=char2, etc), expressed as a percent (divided by 100)
- G = Number of people in group
- Pt = Total probability of at least one member of group getting drop (again, as a decimal)
Pt = 1-((1-(PP1*D))*(1-(PP2*D))...*(1-(PPG*D)))
Therefore, for an item with a drop rate of 5% and a party of 8 people with a prospecting of 100 each:
Pt = 1-((1-(1*.05))8) = 1-((1-.05)8) = 1-(.958) = 1-.6634 see note = .3366 or 33.66%
note: rounding is necessary at this step to keep the number manageable
If there is more than one of a given monster in a group, each monster will have the same chance to drop an item. The math is the same, only now the number of monsters is relevant instead of the number of players.
- Pf = Final total probability of a drop from the given mob by the given group
- Pb = Base total probability of drop from above formula
- M = Number of relevant monster in group
Pf = 1-(1-Pt)M
(this one's simpler because you are dealing with multiple instances of a value that will definitely be the same)
If there are 5 of the monster in the mob, using Pt from the example above:
Pf = 1-(1-.3366)5 = 1-(.6634)5 = 1-(.2920) (rounding again) = .7080 or 70.8%
The prospecting lock is the minimum amount of prospecting that a player or group of players must have before a drop has a chance to occur. If the prospecting lock is not met, there is no chance of getting the drop, no matter how high the drop rate is.
Prospecting locks vary widely. Some prospecting locks are so low that a single player will always have a chance of getting the drop. (In fact, some prospecting locks must be zero because non-paying players are able to get drops while soloing, yet non-paying players have zero prospecting.) Some locks are a little higher but can still be unlocked by a single player who has increased their prospecting through character points and/or equipment. Some locks are so high that they cannot be unlocked by a single player but require a group of players. The highest prospecting locks can only be unlocked by large groups of players. It is the prospecting at the end of the fight that matters, not the prospecting at the beginning.
Some drops have a drop limit. For example, a horn dropped by a monster with only two horns might have a limit of 2. This means only 2 horns may drop from each monster. Even if three or more players succeeded their drop rolls, only the two first receive a horn.
It should also be noted that if a monster drops a horn, wool, and leather, each player can receive only one horn, wool, or leather for each of that monster in the fight. Of course once the drop limit (if there is one) is reached, subsequent players rolling for drops can't get the item anymore.
When the player side wins a fight against monsters, the players may receive drops. Each character gets a drop roll on each drop of each monster, except those drops with prospecting locks higher than the total prospecting of the team. If a drop roll is successful the character receives the item, unless the drop limit for that item has already been reached.
The character with the highest prospecting rolls for items first, followed by the character with the second highest prospecting, and so forth. In the case where two or more characters have the same prospecting, the tie is broken by Initiative.