Does playing several competitions influence a team's league performance? Evidence from Spanish professional football

The sporting performance of professional football teams has often been assessed considering their results in the major regular competition, namely the national league. Here, we show that evaluating league performance without controlling for extra games played in other competitions might produce misleading results. Using Data Envelopment Analysis, we assess the performance of Spanish professional football teams in the League controlling for the extra games played in the King's Cup, Champions League and UEFA Cup. Results show that assessing performance omitting extra games underestimates teams' true performance in the League, the more extra games played leading to greater bias. Consequently, the multioutput nature of football must be considered when assessing team performance.