such as crowd involvement, travel considerations, and environmental factors. The most commonly cited factors of home advantage are usually factors which are difficult to measure and so even their existence is debated. Most of these are psychological in nature: the home teams are familiar with the playing venue; they can lodge in their homes, rather than in a hotel, and so have less far to travel before the game; and they have the psychological support of the home fans. Other factors, however, are easier to detect and can have noticeable effects on the outcome of the game. In American football, for instance, the crowd often makes as much noise as it can when the visiting team is about to run a play. That can make it very difficult for the visiting team's quarterback to call audible play changes, or for any player to hear the snap count. In contrast, the crowd is often quiet while the home team is on offense, and that enables the quarterback to use the hard count intended to draw the defense offsides as the defense can hear the hard count. In basketball, when a visiting player is making a free throw, home fans behind the backboard typically wave their arms or other objects in an attempt to break the visiting player's focus on making the shot. Environmental factors such as weather and altitude are easy to measure, yet their effects are debatable, as both teams have to play in the same conditions; but the home team may be more acclimated to local conditions with difficult environments, such as extremely warm or cold weather, or high altitude (such as the case of Denver teams, as well as the Mexico national football team, many of whose home matches are played in Mexico City). The stadium or arena will typically be filled with home supporters, who are sometimes described as being as valuable as an extra player for the home team. The home fans can sometimes create a psychological lift by cheering loudly for their team when good things happen in the game. The home crowd can also intimidate visiting players by booing, whistling, or heckling. Generally the home fans vastly outnumber the visiting team's supporters. While some visiting fans may travel to attend the game, home team fans will generally have better access to tickets and easier transport to the event, thus in most cases they outnumber