“In the adversarial trial system, a party in a civil case has an absolute right to withhold a source of evidence from the court; but if the issue may determine the case and the opponent’s evidence is strong enough to call for a reply, the failure to call evidence may cause the trier of fact to draw an adverse inference. So, if an obvious witness is not called to testify and no explanation of why the witness is not called is given, the court may infer the witness would prove unhelpful. That said, as the chart below shows, the inference is seldom fully drawn.
A party’s unexplained failure to call an important witness may provide the basis for such an inference. An adverse inference will be drawn against a party only if the opponent is able to establish a prima facie case requiring the party to disprove it or run the risk of losing the case. The inference is that the missing evidence would be contrary to, or at least not support, the party’s case…”