Wait pages

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 to proceed.

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

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 Group function that does the desired calculations. For example:

def set_payoffs(group):
    for p in group.get_players():
        p.payoff = 100

Then trigger this function by doing:

class MyWaitPage(WaitPage):
    after_all_players_arrive = set_payoffs

If you set wait_for_all_groups = True, then after_all_players_arrive must be a Subsession function.

If you are using a text editor, after_all_players_arrive can also be defined directly in the WaitPage:

class MyWaitPage(WaitPage):
    @staticmethod
    def after_all_players_arrive(group: Group):
        for p in group.get_players():
            p.payoff = 100

It can also be a string:

class MyWaitPage(WaitPage):
    after_all_players_arrive = 'set_payoffs'

is_displayed()

Works the same way as with regular pages.

group_by_arrival_time

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 app_sequence like ['consent', 'my_game'], where my_game uses group_by_arrival_time. This means that if someone opts out in consent, they will be excluded from the grouping in my_game.

Presence detection

Note

In oTree 6.0, this behavior has changed. If a participant has not shown any signs of activity within a certain period of time, they are asked if they are still there. If they don’t respond quickly enough, they are sent to an “inactive” page and need to click a button to rejoin.

oTree detects and excludes participants who switched away to another tab or window. In order to be grouped, a user must be active on the wait page. There is a colored dot in the tab of their web page (maybe not visible on mobile). ‘🟢’ means they are considered active. ‘🟡’ means they are considered inactive. We recommend you put some text in your wait page’s body_text to tell participants to be aware of this and stay active on the page.

Notes

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

    @staticmethod
    def is_displayed(player):
        return player.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.)

Notes:

id_in_group is not necessarily assigned in the order players arrived at the page.
group_by_arrival_time can only be used if the wait page is the first page in page_sequence
If you use is_displayed on a page with group_by_arrival_time, it should only be based on the round number. Don’t use is_displayed to show the page to some players but not others.
If 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().

group_by_arrival_time_method()

If you’re using group_by_arrival_time and want more control over which players are assigned together, you can also use group_by_arrival_time_method().

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 function called group_by_arrival_time_method, it will get called whenever a new player reaches the wait page. The function’s second 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 participant.category to each participant.

# note: this function goes at the module level, not inside the WaitPage.
def group_by_arrival_time_method(subsession, waiting_players):
    print('in group_by_arrival_time_method')
    m_players = [p for p in waiting_players if p.participant.category == 'M']
    f_players = [p for p in waiting_players if p.participant.category == 'F']

    if len(m_players) >= 2 and len(f_players) >= 2:
        print('about to create a group')
        return [m_players[0], m_players[1], f_players[0], f_players[1]]
    print('not enough players yet to create a group')

Timeouts on wait pages

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 group_by_arrival_time. Store it in a participant field.

Then define a Player function:

def waiting_too_long(player):
    participant = player.participant

    import time
    # assumes you set wait_page_arrival in PARTICIPANT_FIELDS.
    return time.time() - participant.wait_page_arrival > 5*60

Now use this:

def group_by_arrival_time_method(subsession, waiting_players):
    if len(waiting_players) >= 3:
        return waiting_players[:3]
    for player in waiting_players:
        if waiting_too_long(player):
            # make a single-player group.
            return [player]

This works because the wait page automatically refreshes once or twice a minute, which re-executes group_by_arrival_time_method.

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:

Use `group_by_arrival_time`

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.

Check timeout_happened

You can tell users they must submit a page before its timeout_seconds, 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. First, define a participant field called is_dropout, and set its initial value to False in creating_session. Then use get_timeout_seconds and before_next_page on every page, like this:

class Page1(Page):
    form_model = 'player'
    form_fields = ['contribution']

    @staticmethod
    def get_timeout_seconds(player):
        participant = player.participant

        if participant.is_dropout:
            return 1  # instant timeout, 1 second
        else:
            return 5*60

    @staticmethod
    def before_next_page(player, timeout_happened):
        participant = player.participant

        if timeout_happened:
            player.contribution = cu(100)
            participant.is_dropout = True