# Treatments¶

To assign participants to different treatment groups, you can put the code in the subsession’s before_session_starts method (for more info see before_session_starts). For example, if you want some participants to be in a “blue” treatment group and others to be in a “red” treatment group, first define a color field on the Player model:

class Player(BasePlayer):
color = models.CharField()


Then you can assign to this field randomly:

class Subsession(BaseSubsession):

def before_session_starts(self):
# randomize to treatments
for player in self.get_players():
player.color = random.choice(['blue', 'red'])


For more on how oTree does randomization, see Arrival order & Randomization.

You can also assign treatments at the group level (put the CharField in the Group class and change the above code to use get_groups() and group.color).

If your game has multiple rounds, the above code gets executed for each round. So if you want to ensure that participants are assigned to the same treatment group each round, you should set the property at the participant level, which persists across subsessions, and only set it in the first round:

class Subsession(BaseSubsession):

def before_session_starts(self):
if self.round_number == 1:
for p in self.get_players():
p.participant.vars['color'] = random.choice(['blue', 'red'])


Then elsewhere in your code, you can access the participant’s color with self.participant.vars['color'].

There is no direct equivalent for participant.vars for groups, because groups can be re-shuffled across rounds. You should instead store the variable on one of the participants in the group:

def before_session_starts(self):
if self.round_number == 1:
for g in self.get_groups():
p1 = g.get_player_by_id(1)
p1.participant.vars['color'] = random.choice(['blue', 'red'])


Then, when you need to access a group’s color, you would look it up like this:

p1 = self.group.get_player_by_id(1)
color = p1.participant.vars['color']


For more on vars, see participant.vars.

The above code makes a random drawing independently for each player, so you may end up with an imbalance between “blue” and “red”. To solve this, you can use itertools.cycle, which alternates. :

import itertools

class Subsession(BaseSubsession):

def before_session_starts(self):
colors = itertools.cycle(['blue', 'red'])
for p in self.get_players():
p.color = next(colors)


## Choosing which treatment to play¶

In the above example, players got randomized to treatments. This is useful in a live experiment, but when you are testing your game, it is often useful to choose explicitly which treatment to play. Let’s say you are developing the game from the above example and want to show your colleagues both treatments (red and blue). You can create 2 session configs in settings.py that have the same keys to session config dictionary, except the treatment key:

SESSION_CONFIGS = [
{
'name':'my_game_blue',
# other arguments...

'treatment':'blue',

},
{
'name':'my_game_red',
# other arguments...
'treatment':'red',
},
]


Then in the before_session_starts method, you can check which of the 2 session configs it is:

def before_session_starts(self):
for p in self.get_players():
if 'treatment' in self.session.config:
# demo mode
p.color = self.session.config['treatment']
else:
# live experiment mode
p.color = random.choice(['blue', 'red'])


Then, when someone visits your demo page, they will see the “red” and “blue” treatment, and choose to play one or the other. If the demo argument is not passed, the color is randomized.

Also see Configure sessions.