Crowdsourcing for Participatory Democracies: Efficient Elicitation of Social Choice Functions

December 06, 2016 by David Lee, Ashish Goel, Tanja Aitamurto, Hélène Landemore

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We present theoretical and empirical results demon- strating the usefulness of social choice functions in crowdsourcing for participatory democracies. First, we demonstrate the scalability of social choice functions by defining a natural notion of ✏-approximation, and giving algorithms which efficiently elicit such approximations for two prominent social choice functions: the Borda rule and the Condorcet winner. This result circumvents previous prohibitive lower bounds and is surprisingly strong: even if the number of ideas is as large as the number of participants, each participant will only have to make a logarithmic number of comparisons, an expo- nential improvement over the linear number of compar- isons previously needed. Second, we apply these ideas to Finland’s recent off-road traffic law reform, an ex- periment on participatory democracy in real life. This allows us to verify the scaling predicted in our theory and show that the constant involved is also not large. In addition, by collecting data on the time that users take to complete rankings of varying sizes, we observe that eliciting partial rankings can further decrease elicitation time as compared to the common method of eliciting pairwise comparisons.