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Replicating Pulford & Collman (2008) Experiment on Ambiguity Aversion


Your task is to replicate the experiment reported by Pulford & Collman (2008) showing the persistence of the ambiguity aversion effect irrespective of urn size.
The ambiguity aversion effect, first discussed by Ellsberg (1961), consists in people’s tendency to prefer risky choices when the outcome is 50:50, against choices with unknown outcomes. Imagine you are standing in front of two urns, one containing 50 red and 50 blue marbles and another containing 100 red and blue marbles in
unknown proportions. You are asked to blindly draw a marble from one of the urns and you will win a prize if that marble is red. In such scenario, most people prefer the 50:50 urn and according to utility theory (and intuition perhaps), this means that they deem (a) the chance of drawing a red marble from the ambiguous urn to be less than 0.5, and consequently (b) the chance of drawing a blue marble from the ambiguous urn to be more than 0.5.

The paradox arises when the same decision maker that chose the 50:50 urn is asked to choose an urn for the second time, this time aiming to get a blue marble. It has been shown in multiple studies that the aversion towards unknown outcomes remains, with the majority of people still picking the 50:50 urn, even though according to (b) there’s a higher chance of drawing a blue marble from the ambiguous urn.

In the experiment that you are asked to replicate, the researchers investigated the extent to which the aversion to unknown outcomes is dependent on the urn size, i.e. the number of marbles in the urns. Therefore, they asked three groups of participants to make a single choice between the 50:50 and the ambiguous urn, while varying the number of marbles in each urn between the groups (2,10 and 100). That experiment was conducted by asking participants to physically draw marbles from actual urns and to note down their choices on paper. Your task is to transfer this experiment to a computer program, while trying to replicate the original conditions as close as possible.

Your program should display (at the minimum* the following screens to each participant:
1. Consent page: The experiment should not continue until the participant has expressed his/her agreement - ideally through a checkbox
2. Demographics page consisting at least of age, gender and education level. The program must verify that the participant has provided appropriate values for each field. Otherwise it must display error message(s) – the more precise these messages are relative to the type of error(s) the better.
3. Experiment page (s): See more information below
4. Debrief page.


You are free to include more pages in the experiment.
Note: this image is for illustration purposes only. Critically, the colours of the marbles were not visible during the experiment Regarding the main experiment page(s), you are free to implement it/them in any way you deem
appropriate. In particular, you are free to choose things such as if/how the urns are displayed, how the participant expresses his/her choice and how the result is shown (red marble – lose, blue
marble– win).

There are a few requirements however:
1. The positions of the two urns must be randomized in each trial. While the left urn must always be called “urn A” and the right one “urn B” as in the original experiment, whether it is A (left) that contains the 50:50 or the unknown distribution should be randomly decided every time the experiment runs. Of course, the instructions should also be changing to reflect this.

2. You should be adapting aspects of the experiment (e.g. the instructions) described in the paper to fit the computer-based version that you will be developing. For example, the last part of the instructions found in the paper which reads “You will draw a marble from your chosen urn straight afterwards. I prefer to draw a marble from Urn A/Urn B should be altered to reflect the fact that in the computer based version, a participantchooses the urn through some software means instead of writing something on paper.

However, any change you make must be minimal, since the aim is to replicate the experiment as closely as possible. Critically the description of the process based on which the marbles are placed in the ambiguous urn must stay the same (i.e “….The mixture of red and blue marbles has been decided by writing the numbers 0, 1, 2, . . ., 100 on separate slips of paper….”).

3. Ideally, the processes that (a) compute the distribution in the ambiguous urn and (b) pick marble from the selected urn must mirror the original experiment. For example, although, of course, you won’t be writing numbers on papers, you should be deciding the number of marbles by drawing a sample from an appropriate distribution.

Note: regarding the ambiguous urn you may (a) hand-code the distribution of red and blue marbles based on the numbers reported in the paper (2:0, 8:2, 53:47), or (b) produce a different distribution for each participant.

As in the original experiment, you must write one program for all 3 conditions. As described in the paper, the only difference between the conditions is the number of marbles in each of the two urns (2, 10 or 100).
You should not assume a specific number of participants. In addition, we’d ideally want an equal number of participants in each condition. So, if there are 6 participants, there should be 2 in each condition. Note that you can’t assume that the program will be running on the computer continuously until all subjects have participated. In fact, as is typical in experiments, you should anticipate that your program will be terminated after each participant and will be re-started when the next participant arrives.

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