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In American and Canadian football, a two-point conversion is a play a team attempts instead of kicking a one-point convert immediately after it scores a touchdown. In a two-point conversion attempt, the team that just scored must run a play from close to the opponent's goal line (5-yard line in Canadian, 3-yard line in amateur American, 2-yard line in professional American) and advance the ball across the goal line in the same manner as if they were scoring a touchdown. If the team succeeds, it earns two additional points on top of the six points for the touchdown. If the team fails, no additional points are scored. In either case, the team proceeds to a kickoff.
Various sources estimate the success rate of a two-point conversion to be between 40% and 55%, significantly lower than that of the extra point, though if the higher value is to be believed, a higher expected value is achieved through the two-point conversion than the extra point.
Adoption of rule 
The two-point conversion rule has been used in college football since 1958 and more recently in Canadian amateur football and the Canadian Football League (1975). In overtime situations in college football, the two-point conversion is the mandatory method of scoring after a touchdown beginning with the third overtime.
The American Football League used the two-point conversion during its ten seasons from 1960 to 1969. After the NFL merged with the AFL, the rule did not immediately carry over to the merged league, though they experimented in 1968 with a compromise rule (see below). The NFL adopted the two-point conversion rule in 1994. Tom Tupa scored the first two-point conversion in NFL history, running in a faked extra point attempt for the Cleveland Browns in a game against the Cincinnati Bengals in the first week of the 1994 season. He scored a total of three such conversions that season, earning him the nickname "Two Point Tupa."
Six-man football reverses the extra point and the two-point conversion: because there is no offensive line in that league, making kick protection more difficult, plays from scrimmage are worth one point but successful kicks are worth two. It is also reversed in many high school football and youth football leagues, since there are not often skilled kickers at that level. A variant of this, especially at the youth level, is to allow one point for a running conversion, two points for a passing conversion, and two points for a successful kick.
The Arena Football League has recognized the two-point conversion for its entire existence (in both its original 1987–2008 incarnation and its ongoing revival), allowing for either a play from scrimmage or a drop kick to be worth two points. (The additional extra point for a drop kick is unique to arena football.)
In 1968, leading up to the AFL-NFL merger, the leagues developed a radical "compromise" rule that would reconcile the fact that the NFL did not recognize the two point conversion but the AFL did: the relatively easy extra point kick would be eliminated and only a play from scrimmage would score one point. The rule would be used for the interleague matchups for that preseason, and would not be tried again. Both the World Football League and the XFL revived this concept, making it a point not to institute a two-point conversion rule so as to eliminate the easy kick. What would constitute a two-point conversion in other leagues only counted one point in the AFL-NFL games, WFL, or XFL. However, the XFL later added a rule in the playoffs that allowed the scoring team to score two (or even three) points by successfully executing a play from a point farther from the opponent's end zone (two points if the team could score from the five-yard line and three points if they could score from the ten-yard line).
Defensive two-point conversion 
In American college and Canadian football (as well as, for a significant period of time, the Arena Football League, where missed extra points are rebounded back into the field of play), an intercepted two-point attempt, or one otherwise recovered by the defense, or a blocked extra point kick, can be returned to the other end zone to give the defensive team two points. The team that scored the touchdown then kicks off as normal. This is rare because of the infrequent use of the two-point conversion and the rarity of blocked extra points, and also because of the difficulty in returning the ball the full length of the field. It has proven the winning margin in some games. Only once has an individual player scored more than a single defensive two-point conversion in a game: Tony Holmes of the Texas Longhorns in a 1998 game against the Iowa State Cyclones.
The NFL and high schools that follow the rules of the National Federation of State High School Associations (all U.S. high schools except those in Texas and Massachusetts, which use NCAA rules instead) do not allow this, and a two-point attempt resulting in recovery of the ball by the defense is immediately blown dead and ruled merely as 'no good', although it can, on rare occasions, result in a one-point safety. This one-point safety is the only way a team can have a score of just one point during the course of an American football game. (College and high school football assign a 1–0 score for a forfeited game.) Canadian football, however, allows another one-point play called the single, or rouge.
Conversion safety 
NCAA rules dictate that when a safety occurs during a two point conversion or point after kick, it is worth one point.
Mathematical application of the two-point conversion 
In 2007, blogger Eric Menhart analyzed the value of going for a two point conversion compared to a field goal style extra point in the National Football League, concluding that teams are usually better served kicking the extra point in most cases. This was consistent with the results in the XFL, which had an average success rate of 40% for their one-point conversions (the XFL, as previously mentioned, required scrimmage plays for one point and did not allow kicks). This counters Tuesday Morning Quarterback columnist Gregg Easterbrook's theory that since the average yards gained on a typical scrimmage play is 5 yards, that the opposite is true and that the two-point conversion would, on average, bring a greater point value return; furthermore, Easterbrook cites the Football Prospectus, which says that the average success rate on a two-point conversion is between 50% and 55%, depending on the time frame used and the situations in which the conversion is attempted. The two-point conversion usually involves goal-line defenses and are thus not typical scrimmage plays, resulting in shorter average gains. Regardless of the actual success rate, professional teams seldom attempt the two-point conversion, unless an "eight-point" touchdown results in a certain point margin, either leading, tied, or behind, preferring the near-certain single point (see below.)
While in theory a 50% success rate should result in the same amount of points scored as one-point conversions, this approach does not reflect the realities of game situations. While two-point conversions might result in the same number of points over a season long period this is not how success is measured. Even with a 50% success rate it is certainly possible for a team to miss multiple two point conversions in one game, and then lose to a team who scored the same number of touchdowns but scored their points-after, and while this might be followed by a game where a team makes all their two point conversions to give an average of 50% and the same number of points overall, losing and winning games is much more important for determining success.
There is a relatively common game situation in which the two-point conversion can be an optimal strategy even if its likelihood is under 50%. A team down fourteen points in the final minutes must score two touchdowns while keeping their opponents scoreless in order to tie or win the game. In this situation, it is possible (but unlikely for a team) to go for two after the first score, because if the team makes it, they can kick an extra point in their next score to secure a win, while if they miss, they still have a chance to make the next two-point conversion to get to fourteen. Though the logic seems counter-intuitive, this maximizes a team's win probability. The odds of converting a two-point try either on the first attempt (securing a win) or the second (securing a tie and sending the game into overtime) are higher than the odds of missing both (securing a loss), as long as the expected probability is higher than about 39 percent. Notably, Texas Longhorns coach Darrell Royal successfully used this strategy to defeat Arkansas in 1969's Game of the Century. Nevertheless, coaches continually under utilize this strategy, preferring the conservative approach so as not to alienate fans.
Two-point conversion chart 
This version of the two-point conversion chart was first developed by Dick Vermeil in the early 1970s when he was offensive coordinator under Tommy Prothro at UCLA. The chart is still used by coaches in helping them to determine whether to go for a two point or one point conversion after a touchdown in various situations. The score margin listed in the chart is the margin after the touchdown is scored, but before the conversion is attempted.
|Lead By||Trail By|
|1 Point||Go For 2||1 Point||Go For 1|
|2 Points||Go For 1||2 Points||Go For 2|
|3 Points||Go For 1||3 Points||Go For 1|
|4 Points||Decision||4 Points||Go For 1|
|5 Points||Go For 2||5 Points||Go For 2|
|6 Points||Go For 1||6 Points||Go For 1|
|7 Points||Go For 1||7 Points||Go For 1|
|8 Points||Go For 1||8 Points||Go For 1|
|9 Points||Go For 1||9 Points||Go For 1|
|10 Points||Go For 1||10 Points||Go For 2|
|11 Points||Go For 1||11 Points||Go For 1|
|12 Points||Go For 2||12 Points||Decision|
|13 Points||Go For 1||13 Points||Go For 1|
|14 Points||Go For 1||14 Points||Go For 1|
|15 Points||Go For 2||15 Points||Go For 1|
|16 Points||Go For 1||16 Points||Go For 2|
|17 Points||Go For 1||17 Points||Go For 1|
|18 Points||Go For 1||18 Points||Go For 1|
|19 Points||Go For 2||19 Points||Decision|
|20 Points||Go For 1||20 Points||Go For 1|
- Time "The Two-Point Conversion,", October 6, 1958.
- "CFL History 1970s". Canadian Football League. Retrieved 18 January 2013.
- WiseGeek.com  Definition.
- EricMenhart.com "Going For Two in the NFL: Right or Wrong?,", June 28, 2007.
- Two Point Conversion Chart
- http://www.usatoday.com/sports/football/nfl/2006-11-07-four-downs_x.htm?POE=SPOISVA Four downs: Parcells deals with second-guessing]