Our selection criterion is based solely on the size of the race – we are basically allowing the runners to decide which races we choose as qualifiers. We start with the 30 largest U.S. 100-mile trail races. Then we add established foreign and U.S. trail races of 100-km or more that are of equivalent size.
Races less than 100 miles must combine for a total of at least 6 on the UltraRunning magazine “terrain, surface” ratings. “Established” means no first year races, but a second year event would qualify provided it meets the size standard. Right now that means a race will need to have about 120 finishers to be considered.
Of course this means that there are many worthy races that are not Western States qualifiers – our approach is based solely on the number of runners and not course difficulty or the overall quality of the event. But this has become necessary now that there are more than 150 100-milers in the U.S.A. alone. We re-evaluate the qualifying race list each September.