A 2003 study of US crash data states that driver inattention is estimated to be a factor in 25% to 50% of all police-reported crashes. Driver distraction, a sub-category of inattention, has been estimated to be a contributing factor in 8% to 13% of all crashes. Of distraction-related crashes, cell phone use may range from 1.5 to 5% of contributing factors. However, large unknowns in each category may increase the inaccuracy of these estimates. A 2001 study sponsored by the American Automobile Association recorded "Unknown Driver Attention Status" for 41.5% of crashes, and "Unknown Distraction" in 8.6% of all distraction related crashes. According to NHTSA, "There is clearly inadequate reporting of crashes".