Nanex Research

Nanex ~ 03-Jan-2013 ~ Late EDGX Trades in SPY

See also:
1. SPY - Trades color coded by exchange.
The large black squares are dark pool trades. Note the blue diamonds (EDGX) seem to be following a different pattern.

2. SPY - Trades and quote spread from EDGE and EDGX for clarity.

3. SPY - Trades and quote spread from EDGX.
Now you can see the problem is that trade (black diamonds) from EDGX are simply being reported very late relative to the quote (blue shading). 

Nanex Research