A: While there are no magic words that the expert must say, it is standard and common to hear an expert testify 'with a reasonable degree of medical probability' about his or her opinions. This phrase tells the jury that the experts' opinions are (1) reasonable and (2) describes the relative degree of probability. In medicine there is often very little certainty. As long as the expert can testify that it is more likely than not that something happened, then x, y, or z. The more likely than not standard is a requirement because it establishes that when taking every possibility into account, the event more probably happened than didn't happen- that it's more likely than not that it occurred.