The plaintiff has the burden of proof. Expert witnesses are usually required to testify as to practice standards, while lay witnesses must confine their testimony to facts they perceived with their own senses. To be qualified as an expert, a person must have a sufficient level of education, training, and experience in the relevant field, and it must be shown that their testimony will assist the judge and/or jury in determining a contested issue. The law requires that lay jurors or judges, being untrained in medicine, must accept expert evidence as to whether the provider deviated from the requisite standards but since the plaintiff and the defendant will usually each hire their own experts, there will be conflicting opinions.