Some users of the National Airspace System live by Standard Operating Procedures (SOPs), and some do not. This is arguably the most significant difference between air carriers and general aviation when it comes to training, testing, and cockpit cultures.
This is also, by some measures, a factor in accounting for the differences in accident rates. General aviation, particularly the single-pilot, personal-flying kind, relies not on the use of SOPs, but basic personal minimums for aeronautical decision making (ADM).
