You can check on the NW main page section Flight and Gate Status, plugging in the flight number with the past couple of days to see how it did. Doing this I can see it came in 55 minutes late last night and is expected to be late tonight as well. Late 50% of time sounds like some bad odds to me.

This site might be what you are looking for: http://www.bts.gov/programs/airline_...me_statistics/