Long after the 1998 World Cup was won, disappointed fans were still cursing the disputed refereeing(裁判) decisions that denied victory to their team. A researcher was appointed to study the performance of some top referees. The researcher organized an experimental tournament(锦标赛) involving four youth teams. Each match lasted an hour, divided into three periods of 20 minutes during which different referees were in charge. Observers noted down the referees errors, of which there were 61 over the tournament. Converted to a standard match of 90 minutes, each referee made almost 23 mistakes, a remarkably high number. The researcher then studied the videotapes to yse the matches in detail. Surprisingly, he found that errors were more likely when the referees were close to the incident. When the officials got it right, they were, on average, 17 meters away from the action. The average distance in the case of errors was 12 meters. The research shows the optimum(最佳的) distance is about 20 meters. There also seemed to be an optimum speed. Correct decisions came when the referees were moving at a speed of about 2 meters per second. The average speed for errors was 4 meters per second. If FIFA, football"s international ruling body, wants to improve the standard of refereeing at the next World Cup, it should encourage referees to keep their eyes on the action from a distance, rather than rushing to keep up with the ball, the researcher argues. He also says that FIFA"s insistence that referees should retire at age 35 may be misguided. If keeping up with the action is not so important, their physical condition is less critical. The number of refereeing errors in the experimental matches was ______.