The brain tissue evidence resulted in considerable expense and delay in the proceedings, and you could wonder whether the prosecution was over-egging its pudding. But it didn’t know whether the defence was going to have a ready answer to the DNA presence in the stains. This could be seen as a compliment to the skills of defence counsel, but we could wonder whether, in seeking to prove its case to a certainty rather than merely beyond reasonable doubt, there was (to pursue another culinary metaphor) unnecessary icing on the cake.
What if the appeal had been heard under the new appeal criteria? How would the evidence be analyzed if the court simply had to ask itself whether the error at trial had created a real risk that the outcome of the trial had been affected? The error was material to one chain of evidence: stain – CNS tissue – (mRNA) – Victim1. Taking out this mRNA chain, the remaining chain was: stain – blood – DNA – Victim 1. Did the removal of the mRNA chain significantly weaken the link between the stain and Victim 1?
 Oh alright, for you maths buffs: If the defendant was innocent, a huge coincidence had occurred: blood from each of the two victims was on his shirt. Such little information as there is online about transfer of blood to clothing suggests that about 3 per cent of people have blood from someone else on their clothing. This is much lower than the figures I assumed above. The ratio for the likelihood of the prosecution hypothesis to the likelihood of the defence hypothesis, the probability of finding the match given each hypothesis, would therefore be, for each of the blood sources (ie, the victims), 1 to 0.03, or 33.3 to 1. Combining these gives 33.3 x 33.3 = 1108.89 to 1. Assume, generously to the defence, that before considering this evidence we might expect that the probability of guilt compared to the probability of innocence could be, say, 0.2 to 0.8. This ratio is what is called called "the priors" combined with the other evidence in the case. On the blood evidence, and given this priors-in-combination assumption, the combined ratios are 1108.89 / 4 to 1 = 277 to 1. That is the ratio, given this evidence, of the probability of guilt to the probability of innocence. The probability of guilt would therefore be (converting the ratio to a fraction) 277 / 278 = 0.99. That is proof beyond reasonable doubt, on the basis of the assumption that was generous to the defence. Some people say, but you have just guessed at the priors, the whole thing is too subjective, where is the evidence? The answer is that the priors can vary a lot without having much effect on the result. If, instead of assuming priors combined with the other evidence of 0.2 to 0.8, we assume priors combined with the other evidence of 0.05 to 0.95 (very favourably to the defence, effectively an assumption of innocence), then the resulting probability of guilt after considering the blood evidence is 0.98. That is: (0.05 / 0.95) x 1108.89 = 58.36 to 1, so probability of guilt = 58.36 / 59.36 = 0.98.