Hi bjanssen,Thanks to "Kadaicher1" there is another message...

Hi bjanssen,

Thanks to "Kadaicher1" there is another message board post that describes the history. Some of the links are broken due to Prana's new website structure, but you should be able to find the respective announcements either via the ASX announcements or by digging on Prana's website.

http://investorshub.advfn.com/boards/read_msg.aspx?message_id=92391506

The key bit is:

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To me the big event which allowed this trial to go ahead with less patients was results emerging from the AIBL study. This lecture from Prof Masters explains how few patients they need to prove biomarkers. It is worth looking at it from the 16m mark if you are short of time.
Here is a clip of a Colin Masters lecture. If you scroll to the 23.44 point you will see how many patients he needs to evaluate an AD drug. Based on what they have learned from the longitudinal AIBL study, for a 10% effect, just 23 patients if all have been selected with PET positive AB as have been selected in the PBT2 trial. For a 20% effect, just 7 patients.

http://www.youtube.com/watch?v=oZiPq1Xl59E&feature=relmfu

Note during this lecture he stated the AIBL study now needs a disease modifying drug to try, and the problem is none exist.
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Apart from the fact that the AIBL study allows statistical significance to be achieved with a relatively small "N", I assume that the imaging is really expensive, so the selected number of participants will be a balance between cost and having enough to show significance.