Mendez-Villanueva, A, Mujika, I, and Bishop, D. Variability of competitive performance assessment of elite surfboard riders. J Strength Cond Res 24(1): 135-139, 2010-No previous study has examined the reliability of competitive performance in a subjective (i.e., externally judged) sport such as surfboard riding (i.e., surfing). This reliability is important for the athletes' competitive success prospects and for their support crew interested in strategies or factors that might influence performance. We have therefore determined the typical variation in competitive performance of elite surfers. We obtained official scores (points) for the 11 competitions of the World Championship Tour (WCT). We analyzed 46 male surfers who entered 6 or more such events held in 1 competitive season. To further explore the variability of competitive performance, a separate analysis was performed on official performance points for 182 male surfers who competed on 3 consecutive events within the World Qualifying Series (WQS). Our measure of variability was the typical error (i.e., within-subject variation) expressed as a Cohen effect size (ES) of log-transformed final scores obtained for the surfers after finishing each event. Performance scores in surfing competition showed moderate to large variability. For the 11 WCT events and the 3 WQS events, the ES ranged from 0.72 to 1.01 (n = 46) and from 0.61 to 1.04 (n = 182), respectively. In conclusion, surfers showed much larger variability in performance than previously reported for sports such as running, swimming, or weightlifting. Thus, competition outcomes are largely unpredictable. Considering this large variability in competitive performance, a practitioner monitoring an individual athlete will have little chances of noticing small to moderate changes in competitive performance between consecutive events. Several competitions in a row appear to be needed for tracking the smallest worthwhile performance changes in competition scores as a result of training or other interventions.