Colorado's Court of Appeals recently held, however, that this limitation does not extend to general knowledge, even when the general knowledge is specialized expertise not possessed by the ordinary person, that a juror can recall from memory. As the summary at the Colorado Appellate Blog explains:
[A] juror who was an engineer and used her pre-existing, general knowledge of mathematics (and perhaps physics) to analyze the admitted evidence of relevant locations and distances and the speed of defendant's vehicle did not engage in misconduct. The court held that a juror’s pre-existing personal expertise or knowledge of a general nature – that is, not involving historical or otherwise substantive facts in the case – is not extraneous information which the juror may not use or communicate to other jurors in the course of deliberations.
Historically, when judges were often unskilled lay people (something that is still the case in New York State's Justice Courts), jurors were often selected intentionally because they brought specialized knowledge and expertise to the matter. This is still frequently done when lay arbitrators are used in alternative dispute resolution. But, increasingly, there has been distrust of jurors who go beyond the court record for their determinations. This ruling draws a sensible line between what is permitted and what is not.