Wait pages are necessary when one player needs to wait for others to take some action before they can proceed. For example, in an ultimatum game, player 2 cannot accept or reject before they have seen player 1’s offer.
If you have a
WaitPage in your sequence of pages,
then oTree waits until all players in the group have
arrived at that point in the sequence, and then all players are allowed
If your subsession has multiple groups playing simultaneously, and you
would like a wait page that waits for all groups (i.e. all players in
the subsession), you can set the attribute
wait_for_all_groups = True on the wait page.
For more information on groups, see Groups.
after_all_players_arrive lets you run some calculations
once all players have arrived at the wait
page. This is a good place to set the players’ payoffs
or determine the winner.
You should first define a method on your Group that does the desired calculations.
class Group(BaseGroup): def set_payoffs(self): for player in self.get_players(): player.payoff = c(100)
Then trigger this method by doing:
class MyWaitPage(WaitPage): after_all_players_arrive = 'set_payoffs'
If you set
wait_for_all_groups = True,
then you should set
after_all_players_arrive to the name of to a method on your Subsession model.
In oTree 2.3 and earlier,
after_all_players_arrive was a method,
However, the new format is better and you should use it instead.
Works the same way as with regular pages.
If you set
group_by_arrival_time = True on a WaitPage,
players will be grouped in the order they arrive at that wait page:
class MyWaitPage(WaitPage): group_by_arrival_time = True
For example, if
players_per_group = 2, the first 2 players to arrive
at the wait page will be grouped together, then the next 2 players, and so on.
This is useful in sessions where some participants might drop out (e.g. online experiments, or experiments with consent pages that let the participant quit early), or sessions where some participants take much longer than others.
A typical way to use
group_by_arrival_time is to put it after an app
that filters out participants. For example, if your session has a consent page
that gives participants the chance to opt out of the study, you can make a “consent” app
that just contains the consent pages, and
then have an
This means that if someone opts out in
they will be excluded from the grouping in
If a game has multiple rounds, you may want to only group by arrival time in round 1:
class MyWaitPage(WaitPage): group_by_arrival_time = True def is_displayed(self): return self.round_number == 1
If you do this, then subsequent rounds will keep the same group structure as
round 1. Otherwise, players will be re-grouped by their arrival time
in each round.
group_by_arrival_time copies the group structure to future rounds.)
id_in_groupis not necessarily assigned in the order players arrived at the page.
group_by_arrival_timecan only be used if the wait page is the first page in
- If you use
is_displayedon a page with
group_by_arrival_time, it should only be based on the round number. Don’t use
is_displayedto show the page to some players but not others.
group_by_arrival_time = True, then in
creating_session, all players will initially be in the same group. Groups are only created “on the fly” as players arrive at the wait page.
If you need further control on arranging players into groups, use group_by_arrival_time_method().
Before November 2019, this was a method called
and it was on the Page, not the Subsession.
We recommend switching to the new format.
If you’re using
group_by_arrival_time and want more control over
which players are assigned together, you can also use
Let’s say that in addition to grouping by arrival time, you need each group to consist of 2 men and 2 women.
If you define a method called
group_by_arrival_time_method on your Subsession,
it will get called whenever a new player reaches the wait page.
The method’s argument is the list of players who are currently waiting at your wait page.
If you pick some of these players and return them as a list,
those players will be assigned to a group, and move forward.
If you don’t return anything, then no grouping occurs.
Here’s an example where each group has 2 men and 2 women.
It assumes that in a previous app, you assigned
self.participant.vars['category'] to each participant.
class Subsession(BaseSubsession): def group_by_arrival_time_method(self, waiting_players): print('in group_by_arrival_time_method') m_players = [p for p in waiting_players if p.participant.vars['category'] == 'M'] f_players = [p for p in waiting_players if p.participant.vars['category'] == 'F'] if len(m_players) >= 2 and len(f_players) >= 2: print('about to create a group') return [m_players, m_players, f_players, f_players] print('not enough players yet to create a group')
The above example is hardcoded for only 2 categories (M and F). The below example works for any number of categories. It makes a group as soon as there are 3 players with the same category.
def group_by_arrival_time_method(self, waiting_players): from collections import defaultdict d = defaultdict(list) for p in waiting_players: category = p.participant.vars['category'] players_with_this_category = d[category] players_with_this_category.append(p) if len(players_with_this_category) == 3: return players_with_this_category
You can also use
group_by_arrival_time_method to put a timeout on the wait page,
for example to allow the participant to proceed individually if they have been waiting
longer than 5 minutes. First, you must record
time.time() on the final page before the app with
Store it in
Then define a Player method:
def waiting_too_long(self): import time return time.time() - self.participant.vars['wait_page_arrival'] > 5*60
Now use this:
def group_by_arrival_time_method(self, waiting_players): if len(waiting_players) >= 3: return waiting_players[:3] for p in waiting_players: if p.waiting_too_long(): # [p] means a single-player group. return [p]
This works because the wait page automatically refreshes once or twice a minute,
Preventing players from getting stuck on wait pages¶
A common problem especially with online experiments is players getting stuck waiting for another player in their group who dropped out or is too slow.
Here are some things you can do to reduce this problem:
As described above, you can use
group_by_arrival_time so that only
players who are actively playing around the same time get grouped together.
group_by_arrival_time works well if used after a “lock-in” task.
In other words, before your multiplayer game, you can have a
single-player effort task. The idea is that a
participant takes the effort to complete this initial task, they are
less likely to drop out after that point.
Use page timeouts¶
Use timeout_seconds on each page, so that if a player is slow or inactive, their page will automatically advance. Or, you can manually force a timeout by clicking the “Advance slowest participants” button in the admin interface.
You can tell users they must submit a page before its
or else they will be counted as a dropout.
Even have a page that just says “click the next button to confirm you are still playing”.
Then check timeout_happened. If it is True, you can do various things such as
set a field on that player/group to indicate the dropout, and skip the rest of the pages in the round.
Replacing dropped out player with a bot¶
Here’s an example that combines some of the above techniques, so that even if a player drops out,
they continue to auto-play, like a bot. Just use
before_next_page on every page,
class Page1(Page): form_model = 'player' form_fields = ['contribution'] def get_timeout_seconds(self): if self.participant.vars.get('is_dropout'): return 1 # instant timeout, 1 second else: return 5*60 def before_next_page(self): if self.timeout_happened: self.participant.vars['is_dropout'] = True self.player.contribution = c(100)
- If the player fails to submit the page on time, we set
is_dropoutis set, each page gets auto-submitted instantly.
- When a page is auto-submitted, you use
timeout_happenedto decide what value gets submitted on the user’s behalf.