Announcing z-Tree unleashed

Since its inception in 1998, many researchers have come to rely on the Zurich Toolbox for Readymade Economic Experiments (z-Tree). With online experiments becoming more popular, many researchers have wondered whether z-Tree experiments could be conducted over the Internet, and not merely in the lab. The license terms of z-Tree do not allow the distribution of the binary package to subjects, and it was for that and many other reasons that the answer to this question used to be a resounding “no”.

Few of us would have anticipated that a global pandemic would ultimately make conducting z-Tree experiments over the Internet possible. The closure of our lab that stemmed from the coronavirus posed new challenges to us. Today, I am announcing that the Cologne Laboratory for Economic Research (cler.uni-koeln.de) at the University of Cologne has successfully built a working prototype that allows subjects to safely and securely participate in z-Tree experiments remotely. A GNU/Linux server runs dozens and perhaps even more instances of z-Leaf and one instance of z-Tree. This is accomplished via WINE. The z-Leafs are connected to z-Tree over the loopback interface. The z-Leafs run under distinct GNU/Linux users and they are hence isolated from each other. (We apply several tricks to make z-Tree and the z-Leafs run as smoothly as possible.) Subjects can interface with individually assigned z-Leaf instances over their browser, requiring no additional software. This enables almost anyone to participate and does not violate the license terms of z-Tree.

If you are interested in our setup and our working prototype, we are delighted to announce that detailed instructions will be provided royalty-free within the next months. Before that, we offer to any interested party details regarding our implementation. We are glad to assist you in setting up your own server and enable subjects to remotely participate in z-Tree experiments.