Each NFG experiment is comprised of one or more matches, with a match containing one or more iterations of the game. Within a given match, pairs of subjects are fixed. However, alternative methods of match reassignment are availble. The middle portion of the session parameters tab allows experimenters to select the desired pairing rule.

Note that if an odd number of subjects are connected at the beginning of any match, then one subject is dropped to maintain an even number of subjects. The exception to this is constant pairing, where the odd subject simply waits to be paired with the next subject to connect.

Screen Capture: Session parameters

• Constant pairing: This pairing is not random. Rather, subjects are paired as they connect to the server. Each independent pair automatically begins its first match. Subsequent matches are always played with the same opponent. This option is appropriate for sessions that have subjects connecting at different times, as it is the only option that does not require all subjects to start in unison.
  
  
• One-time random pairing: Once all subjects have connected to the server, players are randomly matched and the first match then begins. Subjects maintain the same opponent in all subsequent matches.
  
  
• Random round-robin pairing with random player role assignment: At the beginning of the experiment subjects are randomly paired such that once a subject has been paired with a specific opponent for a match, the subject will not have the opportunity to pair with the same opponent in a subsequent match until all other opponents have been faced. Further, once a subject has faced all opponents once, subjects are again randomly paired so as to likely avoid the same sequence of opponents. A subject is equally likely to be a row or column player each match.
  
  
• Random round-robin pairing while maintaining player roles: At the beginning of the experiment subjects are randomly split into two groups: row and column subjects. A subject will maintain this role assignment across all matches of the experiment, and will only be paired with subjects assigned to the other role. Further, a subject will be paired with an opponent for a match only once until all other opponents have been faced in subsequent matches. Then, the opponent order is re-randomized and subjects face opponents in a likely different order.
  
  
• Alternate player roles between matches: Each subject alternates between role and column role assignments each match. This option is available only with constant and one-time random pairing modes.
  
  

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