When Brazil kicks off against Croatia in the World Cup on Thursday afternoon, the clock will begin running. It will not stop for fouls or goals; it will not stop even if the president of the Kuwaiti football association decides to storm the pitch. But of course, play sometimes stops, even if the clock doesn’t. To account for this and prevent a team from protecting a lead by running the clock down during these stoppages of play, soccer has stoppage time. A round number of added minutes, usually between one and six, is announced shortly before the 45- and 90-minute mark in the first and second halves. The game will continue for a minimum of that many minutes, until the referee blows his or her whistle to signal the end of the match.

How does stoppage time work, and what are the effects of these rules on the game of soccer?

First, the watchword is discretion. The referee is not bound to any particular calculation of stoppage time. There are general recommendations and best practices that allow fans to project broadly the amount of time added at the end of the match, but no one can ever be quite certain. FIFA Law 7 makes the importance of discretion very clear.

Allowance is made in either period for all time lost through:

  • substitutions
  • assessment of injury to players
  • removal of injured players from the field of play for treatment
  • wasting time
  • any other cause

The allowance for time lost is determined by the game’s fourth official. According to football-bible.com, “Fourth referees  usually have two watches, one which he stops every time there is a stoppage in play (such as injuries, substitutes, and goal celebrations) and another one which he runs to count the time spent in each stop.”

However, the final measure of stoppage time is at the discretion of the referee.

Any events that the referee deems to be taking up extra stoppage time can be timed and added to stoppage time. FIFA’s further instructions on Law 7 add a little bit of precision. Normal stoppages for throw-ins or corner kicks are not to be considered as part of the referee’s calculation, unless of course the referee deems the players to have wasted time. Substitutions, goals and cards are usually expected to count for 30 seconds of added time, but this is no more than a vague “best practices” guideline. When a penalty shot in awarded in stoppage time, typically enough further time is added to allow the penalty to be taken, as happened in the Ghana-Uruguay quarterfinal match in the 2010 World Cup.

Of course, during stoppage time there may be more delays. Indeed, the winning’s team incentive to waste time only grows as the end of the match draws nearer. While the referee is bound to allow a minimum of the announced time to play out, it falls again to his or her discretion to add on further time. The referee’s whistle is then final and ends the game. The whistle tends to blow either at the conclusion of a move or during indirect play. However, there is no rule that establishes this. Referees are not required to allow an attack to continue, but even so it is rare for play to be blown dead in the midst of an attack.

What are the effects of the stoppage-time rule? The big effect is that it adds an important home-field advantage. One of the insights of recent analytical work on sport is that while home-field advantage is quite real, it is probably not primarily an effect of the players feeling more at home, or the crowd inspiring the players. Rather, the primary driver of the home-field effect is the way game officials are swayed by the crowd to call the contest favorably for the home team.

Researchers Luis Garicano, Ignacio Palacios and Canice Prendergast studied the amount of time added to matches in the Spanish La Liga based on the score. They found a huge home-field effect. When the home team was losing by one goal, the amount of added time was about two minutes longer (1.88 minutes to be specific) than when the home team was winning by a goal. Match officials, given great discretion to determine stoppage time, tend to bias their judgments toward the home team.

So if by chance Brazil does find itself down or drawing at the end of regulation time against Croatia on Thursday, do not be surprised to see a big crooked number go up on the board for added time. The home team tends to get these kinds of calls.

Michael Caley writes for Cartilage Free Captain, where he analyzes fancy soccer statistics and bemoans Tottenham Hotspur’s most recent failures. You can follow him on twitter at @MC_of_AMy full World Cup projections and methodology can be found at SB Nation.