You could do something like this
mean = Reap[Sow @@@ Flatten[cogAK6, 1];, _, {Mean[#2], #} &][[2]]; This will be a lot faster than your approach because by using Sow and Reap this code only iterates through the list of data once. In your code, you reiterate through all elements of the data list for every value of gazeNo (so 3000 times instead of only once).
