oTree: Strategic interaction

An Introduction to oTree

Matteo Ploner

The game

A public Goods Game

  • Participants can decide how much contribute to a “public” project out of their endowment
  • Participants are in a group of N subjects (usually 4)
    • What is contributed is multiplied by an efficiency factor $1/N<<1 $
    • What is not contributed is kept in a private account
  • Private incentives are to contribute nothing to the public account
    • But, contributions are efficient
      • \(\Rightarrow\) social dilemma

Treatments: framing

  • We are going to frame the game eiter as a Voluntary Contribution game (VC) or as a Common Pool game (CP)
  • Voluntary Contribution game
    • Participants explicitly decide how much of their endowment contribute to the public project
      • What is not contributed is kept in a private account


  • The interaction is repeated 10x in a partner fashion
    • Total earnings are given by the sum of earnings in each stage
  • Groups of 4
    • Matched together for the entire expeirment (partner)
  • The efficiency factor \(\alpha=.5\)
  • The initial endowment is E=100
    • The individual payoff function is
      • VC
    • \(\Pi_i=E-c_i+\alpha \sum_j^N c_j\)
      • where, \(j\) are the members of \(i\)’s group (\(i\) include)
      • \(\sum_j^N c_j\) is the size of the public project
      • CP
        • \(\Pi_i=c_i+ \alpha (N*E-\sum_j^N c_j)\)
  • Choices are in integer steps
    • \(c_i \in \{0, 100\}\)



  • VC


  • VC


  • CP