I recall reading a study maybe 15 years ago that tried to find a correlation with accident rates in American civil aviation among a number of indicators. Only two stood out. The number one indicator was FAA written exam scores. The authors theorized this could be explained by weak understanding of material or a lax attitude given the non-rigorous nature of the exams. The second was total time on the very low end. Less than 1000 hours if I recall correctly. Unfortunately, I can't find the report. Maybe somebody on the board has access to it..