I got SPX option prices from three different market data sources. In all of them, I can see bid and ask quotes. However, there is only one implied volatility. Does this implied volatility correspond to the bid or to the ask quote? or maybe the mid?
As a side note: the most logical thing to do, would be to ask the sources directly but I can't. Because of the way the option information is presented (it seems to be very similar between the three sources), I think it may be a convection, that's why my question here.