i personally do not think that it is based on any sort of plan... its random i feel... but it looks like they are calling people according to their medical dates..
My friends have informed me that everyone who had their pre employment medicals before the 4th of of december 2013 have been called....
The 8 have their ground classes in june and they leave for their type in august... ( includes all the 32 that have been called till date)