In civil injury cases, such as a car crash, the preponderance of evidence is the standard of proof that will be required for the plaintiff to use in proving liability of the defendant.
This is something that can vary from one jurisdiction to another and from one court to another within the same jurisdiction. The plaintiff in a civil lawsuit must prove their allegations against the defendant using the facts for the case to be successful.
A large portion of civil lawsuits are brought as personal injury claims. In other words, they are based on negligence using tort law.
And they require the plaintiff or injured party to prove specific elements:
The elements of a case, which is the plaintiff’s burden to prove in a civil lawsuit, involves the credible evidence. So it might require documents, police reports, medical documents, expert testimony and witness statements.
This is used as a way of showing the at-fault party caused the injuries and that they had a duty of care. The defendant will have an opportunity to present evidence to combat and controvert the plaintiff’s case. Afterward, or upon objections raised, the judge or jury will weigh the evidence submitted by the plaintiff and defendant to determine if the plaintiff’s case has been proven.
Their job as jurors is to consider the preponderance of the evidence provided and return a decision for the side that has presented a convincing case. The preponderance of proof is the evidence with greater weight in convincing the judge or jury as compared to the case put on by the opposing side.
Even if this tips the scale just a tiny bit, if it is tipped, Plaintiff should rightly win. Success in proving liability in a civil case will mean having the largest percentage produced evidence to sway the jury to make them one side of the case is more believable than the opposing side. Thus, using the rule of evidence if one side convinces the judge or jury by 51% they will prevail in the lawsuit.
However, some juries have been known to make a finding of fault, and awards a penny or "dollar," if they dislike the plaintiff. Thus, the art of legal advocacy, and taking on likable clients, is one of the oft unspoken aspects of a case that is arguably even more important than being able to prove a case.
Many experienced attorneys can look through a plaintiff's case and see into the soul of the victim. If the victim comes off like a jerk, this fact alone could mean a hollow victory.
This is an exceptional sub-issue of proving a case that is worthy of further discussion.
Citations: "Preponderance" - Cornell University: http://www.law.cornell.edu/wex/preponderance