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From Smogon:
The OU tier for DPP and BW is constructed from the league statistics extracted from the past DPP and current BW Pokemon battling medium. These statistics provide the number of times each Pokemon was used in the standard metagame league during each month, and those of the three months prior to the OU tier creation are utilized in particular.
The Pokemon usages from Smogon's Pokemon Online server of the three months prior to the OU update are first extracted. Each Pokemon usage of each month is then divided by the total of the usages during that particular month, so that the probability of that particular Pokemon being used during that month is obtained. Suppose the probability for a particular Pokemon to be used in month m is P_m. These months are ordered in the following manner: if the list is being updated in July, then April, May, and June correspond to months 1, 2 and 3 respectively.
From previous Shoddy Battle statistics, a small computer program was written to generate an equation that would fit best the predicted probability P with the actual probability for each Pokemon. This equation is the following:
P = (20×P_3 + 3×P_2 + P_1) ÷ 24
If, for some reason, a Pokemon was not legal in the Standard metagame in month 1 but was legal in the subsequent two months, its value of P for it would be calculated as follows:
P = (5×P_3 + P_2) ÷ 6
If a Pokemon was not legal in the Standard metagame in months 1 and 2 but was legal in month 3, its value of P would simply be equal to P_3.
After the value of P is calculated for every Pokemon, these are summed up again. This summation S should result in a number that is extremely close to 1. Finally, the Pokemon whose P value is greater than or equal to S ÷ 176.14073324 make the OU tier. This number is the usage that Pokemon require in order to be present in at least one out of every 20 Standard metagame teams on average.
