Writing a pTree app

Creating the app

First, choose a name for your app that is descriptive and short, since you will be typing it and using it frequently. For example, if you are implementing the prisoner’s dilemma, you can choose the name prisoner. If you are implementing the public goods game, you can choose the name publicgoods.

At your command line, run this command, where <appname> is the name you have chosen for your app:

django-admin.py startapp --template http://ptree.org/templates/app.zip <appname>

Once your app has been created, go to settings.py and append its name (as a string) to the list PTREE_EXPERIMENT_APPS.

Writing the app

models.py

(For background, read about Django models here.)

Every pTree app needs 4 core models:

  • Participant
  • Match
  • Treatment
  • Experiment

They are related to each other as follows:

A Participant is part of a Match, which is part of a Treatment, which is part of an Experiment.

Furthermore, there are usually multiple Participant objects in a Match, multiple Match objects in a Treatment, and multiple Treatment objects in an Experiment, meaning that your objects would look something like this:

_images/model-hierarchy.png

Participant

A Participant is a person who participates in a Match. For example, a Dictator Game match has 2 Participant objects.

A match can contain only 1 Participant if there is no interaction between Participant objects. For example, a game that is simply a survey.

A Participant object should store any attributes that need to be stored for each Participant in the match.

Implementation

Participant classes should inherit from ptree.models.participants.BaseParticipant. Here is the class structure:

class Participant
bonus(self)

You must implement this method.

The bonus the Participant gets paid, in addition to their base pay.

code = ptree.models.common.RandomCharField(length = 8)

the participant’s unique ID (and redemption code) that gets passed in the URL. This is generated automatically. we don’t use the primary key because a user might try incrementing/decrementing it out of curiosity/malice, and end up affecting another participant

has_visited = models.BooleanField()

whether the user has visited our site at all

index = models.PositiveIntegerField(null = True)

the ordinal position in which a participant joined a game. Starts at 0.

is_finished = models.BooleanField()

whether the participant is finished playing (i.e. has seen the redemption code page)

Match

A Match is a particular instance of a game being played, and holds the results of that instance, i.e. what the score was, who got paid what.

So, “Match” is used in the sense of “boxing match”, in the sense that it is an event that occurs where the game is played.

Example of a Match: “dictator game between users Alice & Bob, where Alice gave $0.50”

If a piece of data is specific to a particular participant, you should store it in a Participant object instead. For example, in the Prisoner’s Dilemma, each Participant has to decide between “Cooperate” and “Compete”. You should store these on the Participant object as participant.decision, NOT “match.participant_1_decision” and “match.participant_2_decision”.

The exception is if the game is asymmetric, and participant_1_decision and participant_2_decision have different data types.

Implementation

Match classes should inherit from ptree.models.participants.BaseMatch. Here is the class structure:

class Match
is_ready_for_next_participant(self)

You must implement this method yourself.

Whether the game is ready for another participant to be added.

If it’s a non-sequential game (you do not have to wait for one participant to finish before the next one joins), you can use this to assign participants until the game is full:

return not self.is_full()
is_full(self)

Whether the match is full (i.e. no more ``Participant``s can be assigned).

is_finished(self)

Whether the match is completed.

participants(self)

Returns the Participant objects in this match. Syntactic sugar for self.participant_set.all()

Treatment

A Treatment is the definition of what everyone in the treatment group has to do.

Example of a treatment: ‘dictator game with stakes of $1, where participants have to chat with each other first’

A treatment is defined before the experiment starts. Results of a game are not stored in ther Treatment object, they are stored in Match or Participant objects.

Implementation

Treatment classes should inherit from ptree.models.participants.BaseTreatment. Here is the class structure:

class Treatment
sequence(self):

You must implement this method.

Returns a list of all the View classes that the user gets routed through sequentially. (Not all pages have to be displayed for all participants; see the is_displayed() method)

Example:

import donation.views as views
import ptree.views.concrete
return [views.Start,
        ptree.views.concrete.AssignParticipantAndMatch,
        views.IntroPage,
        views.EnterOfferEncrypted,
        views.ExplainRandomizationDetails,
        views.EnterDecryptionKey,
        views.NotifyOfInvalidEncryptedDonation,
        views.EnterOfferUnencrypted,
        views.NotifyOfShred,
        views.Survey,
        views.RedemptionCode]
base_pay = models.PositiveIntegerField()

How much each Participant is getting paid to play the game

participants_per_match

Class attribute that specifies the number of participants in each match. For example, Prisoner’s Dilemma has 2 participants. a single-participant game would just have 1.

matches(self):
The matches in this treatment. Syntactic sugar for ``self.match_set.all()``

Experiment

Not yet documented. You will not be using this object frequently.

views.py

(coming soon)

forms.py

(coming soon)

templates/

(coming soon)

static/

(coming soon)

management/commands/

(coming soon)