Wisconsin has a comparative negligence rule that allows the at-fault dog owner to assert that the injured party was either partly or entirely responsible for the dog attack. For instance, if the dog owner can show the injured person provoked the dog somehow, or was burglarizing their property, then the injured person bears some responsibility for the injury.
In Wisconsin, if the injured party is found to be 50% or less responsible for the injury, the award of damages is reduced by the whatever percentage of fault is attributed to the victim. In cases where the injured person is found to be more than 50% responsible, there is no award for damages.
In cases where the at-fault party is alleging the victim is partly responsible for the attack, it all comes down to evidence. A good attorney can make mincemeat of a fraudulent claim that the victim did something to provoke the attack.