It’s up to the jury to decide fault in a personal injury case. They listen to all of the evidence, and they determine each person’s fault by a percentage. When the jury begins their deliberations, the judge gives the jury instructions about comparative negligence.
The jury reviews the case and apportions the damages by a percentage to everyone involved. The jury doesn’t have to assign a percentage of fault to anyone. Instead, they’re free to decide that someone is entirely at fault and that someone else isn’t at fault at all. In most cases, their decision is final.