Here's the closest thing I've found to an explanation of how
to set up and conduct a prediction market. This paper, INFORMATION AGGREGATION
MECHANISMS: CONCEPT, DESIGN AND IMPLEMENTATION FOR A SALES FORECASTING PROBLEM,
by Charles R. Plott of CalTech and Kay-Yut Chen of Hewlett Packard
Laboratories, describes how they set up a prediction market for sales forecasts
at HP with the following results:
·
In 6 out of 8 events for which official forecasts
were available the market predictions were closer to the actual outcome than
the official forecast.
·
The probability distributions calculated from market
prices were consistent with actual outcomes.
·
The market made accurate qualitative predictions
about the direction that the actual outcome will occur (above or below)
relative to the official forecast.
I’m separating my notes into two posts. First, the nuts and
bolts about how it was done and second, some of the scientific issues.
How it was Done
The Prediction
Typically, the
prediction was for monthly sales for a month three months in the future.
Business Constraints
·
Hesitation
to engage employees in an exercise in which they might lose money. Solution: provide
a small amount of cash to each participant before the market sessions - this
constrains the amount of stakes a participant can have in the market and affects
incentives to trade.
·
Market
has to offer useful information. E.g., if forecasts are not valuable if they
are made with horizons less than 3 months then market sessions need to be
conducted 3 months before the event to be predicted.
Who participates
·
Relatively small number of participants chosen.
Selected specifically from different parts of the business operation because
they were thought to have different patterns of information about the targeted
event. These patterns of information, including market intelligence, specific
information about big clients, and pricing strategies, were in need of
aggregation. No public summaries of information available to the participants
during the operation of the IAM. The official forecasts were not known until
after the IAM closed.
·
Participants need to be selected carefully –
don’t want to “miss” a person with much information but it might not be
efficient to include many people without any relevant information. Little is
known theoretically about the information size relative to the market that
might be required for effective information aggregation to take place.
·
Laboratory
experiments have suggested that a small number of uninformed participants
provide both market liquidity and a function of adding “consistency” to the
market through a process of “reading” and “interpreting” the actions of others.
So, around five subjects was recruited from HP Labs (with little or no
information) in each experiment.
Preparing the participants
15-20 minute
instruction session from: explained the structure of incentives, the market
mechanism and the web interface. In addition, the participants were told the
goals of the experiment and were told that their participation was important
for HP business. Contact information provided and participants were encouraged to
call if they encountered difficulties.
Defining the
Contracts to be Traded
Most similar to the IEM “winner take all” markets (state
contingent securities).Traded a
complete set of state contingent contracts (Arrow-Debreu securities). The space
of possible outcomes was partitioned into about 10 intervals. Each interval was
given a name and with each interval there was an associated security with the
same name that traded in a market with that name. Thus the interval 0-100 would
be associated with a security named 0-100 that traded in a market named 0-100.
The interval 101-200 would be associated with a security named 101-200, etc.
Payoff
If the final outcome
fell in an interval, the corresponding security would pay, say, one dollar per
share at the end of the experiment. All other securities would pay nothing. A
higher payoff per share would place more value on the share but the payoff per
share interacts with the total cost of the exercise and the potential volume of
trades and related market liquidity.
How to Start
Each participant is given
a portfolio of shares in markets and cash to start. Could start with equal
shares in all securities or could start with shares in every other security,
alternating which security was first across participants. The unequal
distribution of endowments was used to encourage trading by attempting to make
sure that the initial endowments of securities did not approximate the ultimate
equilibrium.
Market Mechanism
Web based, double
auction markets. Marketscape software (Laboratory of Economics and Political
Science at Caltech.)
·
All the
markets for an event were organized on a single web page for easy access.
·
Links to
a complete time series of trades available.
·
Links available
to HP data bases, which allowed participants to review data held by HP.
·
A
participant could enter a buy offer, a sell offer or acceptance of an offer
through the web form on the page. Orders were compared to the other side
immediately. If a trade was possible, it was executed and if not the order was
placed in an order book. The best offers were listed on the main market web
page. The whole book of offers was available for each market at the click of a
button.
·
Participation
was anonymous. However, each participant was assigned a subject ID number for
each experiment. During the experiment, the subject ID number of the person who
made offers and transactions were public knowledge. Participants had the
ability to track behavior of other subjects with in the same experiment if they wished to.
When should market be open?
·
In all
cases the information was gathered for a week with the markets being open
during lunch and in the evening every day. Management did not want participants
being preoccupied with the task during the working day when pressing issues needed
attention.
·
It is
desirable to have a schedule (for example, 24 hours for a week) to minimize
conflict with other activities.
·
It is
not desirable to leave market open for long periods because participants will
often find a lack of activity in the market and thus lose interest.
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