CMKM Diamonds Inc. (fka CMKX)

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Actually, the block size has nothing to do with the categorization. It's based on the brokerage firm used. If the firm typically trades major mutual funds and portfolios, then any trades coming from that firm automatically go into the "institutional" category even if all the buyers are retail. A number of these firms are also MM's. In such cases, it's impossible to tell whether a trade from them is for a client or for their own trading account. So, a lot of MM activity also gets lumped into "institutional".

Anyway, I agree it's not a good indicator that big institutions are trading. Most of them steer far, far away from sub-pennies.