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Deals damage. The Base Power of this move increases depending on the number of stat boosts accumulated by the target, as per the following calculation:
Base Power = 60 + (20 * Total Number of Stat Rises)
For instance, if the target had 1 Swords Dance boost and 2 Calm Mind boosts, then this would be +2 Attack, +2 Special Attack, and +2 Special Defense, so Punishment's power on it would be 60 + (20 * 6) = 180. This move's maximum power is 200. Also, stat drops do not affect the Base Power.
Punishment is an interesting move because it can serve as an immediate stop to users of setup moves, in particular Calm Mind. Choice Scarf Ho-oh is the main user of this move in Ubers, while Choice Scarf Primeape does a similar job in RU. Regardless, the minimum 60 base power is not too bad, either, since it helps Primeape in particular against Ghost-types like Cofagrigus and Rotom.
Crunch is a more powerful option that does not vary in Base Power based on anything.