Background Even though the Chinese Super League (CSL) introduced Video Assistant Referees (VARs) in the 2018 season, it did not intervene in every game. The Hawthorne effect highlights how human behavior can change when individuals are aware they are being observed. VARs monitoring means VARs just introduces but does not intervene in the games. Thus, this research compared the performance in the games where VARs did not intervene and without VARs, to investigate whether VARs monitoring influences match performance, similar to the Hawthorne effect. Methods The sample includes matches in which VARs were not applied in the 2014-2017 season (n = 1920) and in which VARs were not intervened in the 2018-2021 season (n = 528). The Generalized Linear Mixed Model was used to examine the different variables in different groups. Results The study found that after the implementation of VARs in the game, the percentage of non-intervention was 32%. Even though VARs was not applied, it had a significant impact on goals (p < 0.01, Cohen`s d = 0.04), penalties (p < 0.001, Cohen`s d = 0.07), free kicks (p < 0.001, Cohen`s d = 0.18), tackles (p < 0.001, Cohen`s d = 0.13), fouls (p < 0.01, Cohen`s d = 0.11), and red cards (p < 0.001, Cohen`s d = 0.04). Still, there were no significant differences in the number of shots, passes, corners, and yellow cards (p > 0.05). In addition, the effect size of all significant variables shows a small effect. Conclusions In the CSL, VARs intervene frequently in matches. VARs monitoring also had a measurable, albeit small, impact on sports performance, especially defensive performance. In addition, VARs monitoring follows the idea of FIFA to encourage attacking, and it also benefits limited fouls.
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