From my admittedly non-extensive experience of sitting on one jury:
You'd think it would be easy to predict what will happen in the Guy murder trial - ie, if the media reports are accurate there isn't much in the way of evidence that the jury can consider beyond reasonable doubt, so they should be back reasonably quickly with a not-guilty verdict.
However, they do need to sit down and work through that evidence and see if they are all agreed about the difference between "most likely" and "beyond reasonable doubt," and which of those categories the various bits of evidence come into, so two days wouldn't be unreasonable in a case like this.
Of course, human psychology being what it is, there's no guarantee the lack of solid evidence will result in a not-guilty verdict. Some people don't take a very rational approach to issues and can be unshakable in a belief they can't defend with persuasive or logical argument. And perhaps more to the point in this particular case, there can be a temptation to use balance of probabilities rather than beyond reasonable doubt for your verdict. New Zealand's double jeopardy law means that finding Ewen Macdonald not guilty equates to declaring him innocent - he can never be tried again for this crime, even in the unlikely event that better evidence turns up. If a juror's of the view that Macdonald most likely commited the murder (which wouldn't be surprising, as he most likely did), voting 'not guilty' might go very much against the grain.
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