The introduction of video assistant referee technology in Portugal's Primeira Liga reduced effective playing time by up to two minutes per match, according to a study by NOVA Information Management School released on Saturday.
The academic investigation analyzed 1,616 matches across Portugal's top two professional football divisions played between the 2015-2016 and 2018-2019 seasons. Researchers found that the reduction in effective playing time occurred immediately after the technology was implemented and persisted across subsequent seasons.
According to the study, defended in October 2025 by Luís Filipe Santos for his Master's degree in Data Science and Advanced Analytics, the percentage of effective play in the top tier dropped from 52.77 percent (47 minutes and 30 seconds) before video refereeing to 51.46 percent (46 minutes and 19 seconds) after its introduction in the 2017-2018 season.
The research was supervised by Bruno Damásio, a professor and researcher at NOVA Information Management School, a constituent school of Universidade NOVA de Lisboa that specializes in information management and data analytics.
Methodology and control group comparison
The study compared the final two seasons before video assistant referees were introduced in Portugal (2015-2016 and 2016-2017) against the first two seasons with the technology (2017-2018 and 2018-2019). The estimated reduction in effective play ranged between 1.13 and 2.17 percentage points, representing a loss of one to two minutes per match.
To isolate the effect of video refereeing from other factors that alter match pace, the research team used the Difference-in-Differences econometric methodology, inspired by the work of Nobel Memorial Prize in Economic Sciences recipient David Card. The second-tier Segunda Liga served as a control group because it did not adopt video refereeing until the 2023-2024 season, maintaining stable play duration during the study period.
The specific analysis of the Primeira Liga examined 1,060 matches, comprising 516 games before implementation and 544 games after. Damásio explained to news agency Lusa that researchers reconstructed a counterfactual model of the top division without video refereeing to calculate the impact credibly.
Persistent loss of playing time
Damásio highlighted that no adaptation period was observed within the analyzed window. He noted that while an initial shock during the first year might have been expected to diminish over time, the reduction appeared during the first half-season with video refereeing and remained unchanged throughout subsequent periods.
Damásio stressed that losing one to two minutes per match creates a substantial cumulative impact across an entire championship. With each team playing 34 league fixtures a season, the lost time totals between 30 minutes and one hour of live play per club annually.
Given that an average match contains approximately 45 minutes of active play, Damásio noted that each team effectively loses between three-quarters of a match and one and a half matches per season. He calculated that a club competing in 34 matches essentially plays only 33 full matches of football. Damásio added that while a one-minute loss is imperceptible to spectators in the stadium, accumulating ten hours of lost ball time per season across an industry built on live sport represents a significant loss.
Proposed solutions and decision split
Speaking on the sidelines of the study conclusions, Damásio called for greater data transparency and continuous evaluation of football regulations. He argued that effective playing time should be an official competition indicator published round by round and team by team, observing that Portuguese football currently debates match duration based on impressions rather than figures.
Damásio pointed to regulatory tests authorized by the International Football Association Board in English football as potential measures to reduce stoppages. These include strict time limits for restarts and temporary numerical reduction rules while injured goalkeepers receive medical treatment on the pitch.
Damásio also recommended separating purely objective decisions from those requiring referee interpretation. He stated that factual, binary decisions such as offside calls or whether a ball crossed the goal line do not require stopping play to consult a monitor for a minute, as automated sensors can resolve them within seconds. Conversely, subjective judgment calls like penalty decisions cannot be resolved by technology in the same manner.
Video assistant referee rules and protocol
The video assistant referee is a technological system designed to support on-field match officials by reviewing video footage to correct clear and obvious errors or serious missed incidents during matches.
Under international protocol established by the International Football Association Board, the governing body responsible for determining the Laws of the Game, video referee intervention is strictly restricted to four match-decisive scenarios: goal validation, penalty kick decisions, direct red cards, and cases of mistaken player identity.
