LIONESS experiment

A standard LIONESS experiment can be created in LIONESS Lab and brings along a set of standard features to enable online interactive experiments. The build-in features are described here. They can be configured when developing an experiment.


This section provides an overview over the general functioning of a LIONESS experiment. It does not discuss the details of how to implement these things. The details can be found in the develop section.


For online experimentation (in contrast to normal lab experiments) some principles should be considered which are all implemented in LIONESS expeirments.

  1. Minimize dropouts if possible!
  2. The basic principle for all this is that participants should be kept active and waiting times should not be too long.
  3. Dropout of participants are unavoidable. Measures should be in place to deal with this.
  4. Matching and assignment of roles and treatments can only be done on-the-fly and not predetermined as the total number of participants varies.
  5. Participants should be able to complete the experiment even if their couterparts dropped out in order to prevent negative feedback by participants which damage the experimenter’s reputation.

Experimental Flow

The following figure shows the standard flow of an online experiment and describes all the mechanisms along this flow.

  1. Participants log in via a link from an external website (e.g. Amazon MTurk).
  2. Then they go through the experiment.
  3. At the end they return to the external site to get their payoff.

The blue arrows show the normal flow of the experiment. The red arrows show possible dropouts. The euro symbol shows if a showup fee is paid on dropout or not. The blue line around the stages mark the boundaries of a LIONESS experiment.

When participants arrive from the external site, they are registered and get a player number. Then they go to the first stage of the experiment. After that a quiz may follow where, participants have to answer control questions in order to participate. The quiz is optional. After passing the quiz, participants wait in the lobby to be matched to a group. This is optional for single player games. Then participants make their decisions (maybe over several rounds) before they are informed about their payoff (end). From there they are directed back to the external site they came from.

On the way through the experiment, LIONESS experiment handle all kind of possible dropout issues, which are described in the following.


On dropout participants receive different standard messages, which can be customized in the paramters. For all list of all messages see also here. All messages refer to the terminology HIT which is a task on Amazon MTurk.


LIONESS experiments do their best to prevent double participation by IP address check and cookies in the browser. Some of these measures can still be circumvented with some effort. If you want to be 100% sure that participants only participate once they should be provided with a ticket or unique ID which they have to enter during the experiment.

a) Internet Explorer

LIONESS experiment (and many other modern web applications) do not support the Internet Explorer as it is outdated. Experimenters should inform their participants that they cannot participate if the use the Internet Explorer. If participants use the Internet Explorer, they are informed that they cannot participate in the experiment with the following message:


As indicated in our HIT text on MTurk, our HIT does not support Microsoft Internet Explorer. Please return this HIT. We apologise for any inconvenience caused.

Participants can return with a different browser as they were not registered with their IP address in the LIONESS experiment yet.

b) Task not active

As an experimenter you can set the task inactive or active at any time in the control panel. If the task is inactive, new participants cannot enter and receive the following message:


This HIT is currently offline. You cannot participate at this time.

Participants who are already in the game can complete the game.

c) Double login

LIONESS experiments record the IP addresses in an anyonimized way to protect personal data. With the anyonimized IP addresses it can be checked if two participants log in from the same IP address. The actual IP address cannot be retrieved.

If a second participant tries to log in from the same IP address, he or she receives the following message and cannot enter.


According to our records, your device has already been connected to the server during this session. Participants are only allowed to enter a session once. Thank you for your understanding.

The IP address check is deactivated whent the test mode is on.


If you think that your participants may use the same IP address you may switch to test mode. Otherwise they cannot enter. This may happen if e.g. all participants play in the same network.

d) Session full

In the paramters the total number of players can be specified. If enough players entered the game, further participants cannot enter anymore and receive the message:


We have sufficient participants for this HIT. Unfortunately, you cannot participate at this time. Thank you for your understanding.

If you increase the total number of players during the experiment, more participants are allowed to enter.


The number of participants are counted at the beginning of the experiment. It also includes participants who started the game but dropped out according to reasons f), g), i), j) and k). This means you should choose a number that is larger than the acutal number of participants that you need.

e) Not registered

If a participant tries to participate in a LIONESS experiment by navigating to a stage in the experiment without being registered he or she is informed about that.


You are currently not logged in. You cannot participate in the HIT.

Entrance to an experiment is only possible via the first stage where participants are registered.

f) Time out

In each stage, you can define a maximum time participants have to complete the stage. This is useful to keep up the pace of the experiment. If a participant does not finish in time, he or she can be directed towards a different stage in the game or to the standard time out page which shows the following message:


You did not make a decision before the time was up. You have been removed from the HIT. You can close down this window.

g) Kicked out by experimenter

In the control panel, experimenters can kickout participants by entering their player number. They receive the following message and get their show-up fee. The values between $ signs are filled by the values set in the parameters.


Unfortunately, this HIT was terminated for a technical reason! You cannot continue. You will receive your guaranteed participation fee of $ $participationFee$. To collect your earnings, please fill out this random code on MTurk: $randomid$ Once you have filled out this code, you can close this window. Thank you for your participation.


This features should be used with care. It is mainly intened when technical problems appear.

h) No re-entering possible

If participants try to re-enter after being kicked out, they are also informed that they cannot participate in the experiment anymore.


You are currently not logged in. You cannot participate in the HIT.


This information that a participant has been kicked out is based on the IP address (if the test mode is switched off) and a cookie in the browser. If the participant uses a different browser from a different IP address he or she can still enter as a new participant.

i) Too many quiz errors

In the quiz stage, the experimenter can specify a maximum number of quiz failures. It the participant fails more than that, he is excluded from the experiment and receives the following message:


You did not answer the quiz correctly and were excluded from further participation.

j) No group match

In the lobby, participants wait until they are matched for a cetrain time span. If there is no other participant within this time span, the participant is directed to a page where he or she can choose to wait additional two minutes or to leave the experiment. In the latter case the participant should receive the showup-fee. The experimenter can set to which stage the participant is directed when he or she leaves.

k) Group aborted

In the parameter setting the experimenter can choose what happens if during the decision phase a participant drops out. If the experimeter opts for terminate group, all players of the group are kicked out of the experiment and receive the following message:


Unfortunately, one of the players in your group dropped out of the HIT! You cannot continue. You will receive your guaranteed participation fee of $ $participationFee$. To collect your earnings, please fill out this random code on MTurk: $randomid$ Once you have filled out this code, you can close this window. Thank you for your participation.

For the different options on drop outs in a group see Reference manual > Paramters > Predefined parameters > dropout handling.