If one’s injury is caused by the combined negligence of multiple tortfeasors, liability will be determined according to one of several tests.
Substantial Factor Test In cases where the negligent actions of each tortfeasor alone would have caused the entire injury by itself and the harm is indivisible such that damages cannot be apportioned among the tortfeasors, each is liable for causing the entire harm. To be liable, a defendant’s negligence must have played a substantial part in causing the harm.
Multiple Negligence In cases where the negligent actions of each defendant would not have caused the injury alone, each is liable for the damage each defendant actually caused.
If it cannot be determined which of the two negligent actors caused the injury, both are liable unless one can prove his innocence.
Enterprise Liability Theory Each member of an enterprise is liable for damages caused by the enterprise as a whole when there has been close cooperation among the different members and damages cannot be apportioned.