Question about Groups and supplying separate stimuli to each participant

Hello,

In theory yes, you could define as many different groups in the CSV as you’ll have participants. Two problems though are ensuring 1) that each participant is really assigned a different group than any other participant, and 2) that you really have the number of participants you expect.

Practically speaking, you can try to address the first point by incrementing the counter as soon as the experiment page opens. It would still take a few milliseconds though, so you might end up with some participants being assigned the same group if they take your experiment at the same time. This problem is really hard to avoid.

The second point is particularly problematic when using recruiting platforms like MTurk or Prolific. The problem here is that several people will start taking your experiment and quit before the end. If you’re incrementing the counter whenever someone opens your experiment, those participants will mess with group assignment.

The ideal solution would be to recruit all your participants first, assign an individual group to each of them, and then run the experiment. I don’t know if that’s an option for you. Or if you can afford it, you can run one participant at a time.

Remember that you can force a group in the URL by using this format: https://expt.pcibex.net/ibexexps/example/example/server.py?withsquare=0 (the 0 here means “run the first group”—use 1 for the second, 2 for the third, etc.)

Jeremy

Hi Nickolas,

You currently can only send results once per participant, so chances are you are seeing this message because your experiment attempts to send the results more than once.

I might implement an option in the future to regularly send results to the server over the course of the experiment, as you suggest, but that would require revising how the server handles result-saving requests, which I can do on the PCIbex Farm but not on the original Ibex Farm, of course, and for now I am trying to keep PennController as compatible with the original farm as possible.

Jeremy

Hi Nickolas,

I hadn’t noticed that the results were sent anyway. You should have the same MD5-hased IP each time the results are sent, but a different ReceptionTime, so you won’t be able to use those to discriminate redundant results lines from the same participant form results lines from two participants taking your experiment on the same machine with the same browser. If you log a uniquely identifying ID otherwise then that’s not a problem.

Also keep in mind that redundancy might make your results files grow quite fast, which could be a problem depending on the situation. The size of your results files counts toward calculating your quota on the PCIbex Farm, for example.

All that being said, you can add this bit of code to your script to disable the specific prompt:

const oldAlert = window.alert;
window.alert = function(message) {
    if (message=="WARNING: Results have already been sent once. Did you forget to set the 'manualSendResults' config option?")
        return;
    oldAlert.call(this,message);
}

Jeremy

Hi Nickolas,

You won’t be able to specify a different filename for the results file as all the results are sent to the same address, and storing them in the results file is handled on the server side entirely.

However it should be simple enough to look at the lines from your results files that correspond to your last trial so as to list all the participants who made it to the end of your experiment, and then filter the results using that list.

Jeremy