Quote from TheStudent:
An analysis of Gap trades is not complete without an assessment of the risks involved.
% of Gaps being filled x size of Gaps = reward opportunity
If time is being used to determine % of Gaps being filled (eg, end of day, first 30 minutes, after x days etc), a time stop would make it easiest to analyze Gap strategies.
Down Gaps have a higher fill rate if you allow for multiday holding periods, because you have the undercurrent positive drift of the S&P working in your favour.
Gaps come from the interaction between the overnight session and the day session, so you cannot understand gaps without looking into the overnight session.
An interesting way to look at Gap trades is to set up your favourite charting program to display the night session only. You get these weird looking charts - it bit like rethreading an old Pet Shop Boys cassette tape and listening to it backwards ...