[chollida1]

Well the truth is that for some firms it is for about 99.9% of all trades.

Renaissance Technologies founder Jim Simmons is famous for saying they didn't override the algorithm.

In practice most HFT firms do a mix of both. The algos will do the vast bulk of the trading but you have human traders monitoring algos to do clean ups for cases where the algo gets "stuck". What defines "stuck" really depends on the sophistication of the algo and the firm itself.

Some algo's, such as internalizers's for crossing bought flow are so simple that there probably doesn't need to be much over site at all.

Market making is very similar, with the exception of a flash crash where they might pull out, market making algos should just run themselves.