In rugby union, little research has concerned itself with yellow cards and the effect it has on the team’s performance. The aims of this study were twofold: To improve the lack of research surrounding yellow cards in rugby union; and to investigate how critical a yellow card is to a rugby union teams’ performance. A sample of twenty matches from both the 2016/17 and 2017/18 Aviva Premiership were selected for the analysis. Only close matches were included (less than two converted tries in points difference). Three conditions were created to see how performance differed across the course of the game: ten minutes pre-yellow card, ten-minute sin-bin, and the remaining sixty minutes was scaled down to a ten-minute period. Key performance indicators were split into various categories including: scoring indicators, possession, set piece, defence, attack, and discipline. Sportscode Elite was used to notate instances, with data analysis through averages and paired samples t-tests conducted through SPSS. The most interesting significant effects include (p < 0.05): score line negatively decreased post-yellow card (t19 = 2.809, d = 0.424, p = 0.011); less points were scored and more points were conceded while the penalised team have a player in the sin bin (t19 = -2.871, d = -0.910, p = 0.010); average time in the oppositions
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twenty-two meter was significantly greater for the remaining sixty minute condition than the pre-condition (t19 = -2.589, p = 0.018) and the during-condition (t19 = -3.652, p = 0.002); possession was kicked the most in the during condition; more negative passes were thrown in the remaining condition than both the pre-condition (t19 = 2.939, d = -0.842, p = 0.008) and the during-condition (t19 = -2.269, d = -0.748, p = 0.035); fewer attacking errors were made during the ten-minute sin-bin in comparison to the remaining condition (t19 = -2.707, d = -0.527, p = 0.014). The results highlight a surge of defensive pressure prior to the penalised team losing a player to a yellow card, with further work now required to advance the applied benefits.