Statistics confirms what football supporters suspected, Carlos Lago (GEN Uvigo) and Maite González (European University of Madrid) have made a study where they analyze all La Liga matches during the 2014-2015 season. They have found that the greater the difference in the score, less additional time is added to the end of the game.
In their study "The Influence of Referee Bias on Extra Time in Elite Soccer Matches" published in the “Perceptual and Motor Skills” journal, the researchers have found that referees give advantage to the big teams by shortening the added extra time when they are ahead on the scoreboard. However, If they are losing the referees would slightly lengthen the game.
Their study was based on 380 matches played in the Spanish First League, considering other variables (goal difference, number of red and yellow cards, changes, assists and fouls). After applying linear regression it was observed that the greater the difference in the score, the lower the additional time was granted by the referee.
There is also a bias that favours local teams, as the pressure of spectators manages to make the referees to add up to 112 seconds to the usual extra time in the cases when the home team is losing.
According to the interview with Carlos Lago, these actions have not been shown to be malicious, but simply human decisions. To avoid this, he recommends better training of referees and avoiding subjectivity.
The full newspaper article on this topic is available at the following link: