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the reason they fell back from 42% to now 39% is because there's been a few miners pulling out for the reason of protecting the bitcoin network.
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I haven't yet checked, but at a guess I'd say that could easily be due to variance. A larger drop over a longer time period might be indicative though.
what about that ever growning 'unknown'? That could work with ghash to do the 51% deed correct?
I do my own tracking, and unknown is only about 80 blocks per week. A good deal of it uses the same gen address, so it's unlikely that ghash would use that if they were being tricky.
why is so much hashing listed as unknown anyway?
Because whoever it is that is doing the tracking your referring to isn't using all possible data. I'm using coinbase sigs, known gen addresses, and pool websites (where available). You put it all together and you get better tracking than blockchain.info.
For example I can track Deepbit properly, and also bitparking and some others that confuse blockchain.info. It also means that if an unknown has used the same gen address for a long time, I can track that too. There are known unknowns (gen address and nothing else) and unknown unknowns (no coinbase sig and no record of gen address being used more than once). In total, all unknowns rarely get as high at 10%, although this may have changed recently.
Once I finish the current upgrade I'll post more accurate block stats somewhere.
Sorry for derailing thread, but huge part of unknown is 100th mine (currently hashing at about 500TH/s)