About 2 weeks ago I posted a chart showing the volume distribution (cumulative) over one trading day. I looked again at the data to see the lot size being traded. There were a total of 2,117 trades made on that day.
There is no discernible pattern to the trade size (at least that I can see). The trades were also made at different intervals, with 267 different intervals being used, varying from 6min 44 secs to 1 sec. There were, in quite a few cases, however, many trades performed at exactly the same time. Again the trade size appearred random.
The pure number of trades being made, and the frequency does suggest that there is some form of automation, but I cannot see a distinct pattern, especially as the total daily volume also varies. No doubt there is an algorithm somewhere that takes various factors into account to determine trade size, and when it is made. It is also possible that somebody defines a target volume, which they try to get as close as possible to, but the total daily traded amount does have a quite wide variation.
I also suspect that different Companies approach their buybacks differerntly, so it is difficult to make a general conclusion on the philosophy behind the individual purchases.