Research Seminar in Decision and Action Theory: Christian List (MCMP)

Title:

Aggregating expert judgments on non-binary issues

Abstract:

Suppose that a group – e.g., a committee or expert panel – is making judgments on some issues, such as climate variables or economic variables, with potentially complex interconnections between them. Suppose further that at least some of those issues are non-binary, i.e., they are not just yes/no-questions but include variables that may take many values, such as meteorological or macroeconomic variables that fall into a continuous range. How can the group arrive at “intelligent” collective judgments on the given issues, based on the group members’ individual judgments? We investigate three challenges raised by such “non-binary judgment aggregation”. First, reasonable methods of aggregation (such as defining the collective judgment for each issue as the average or median judgment) can produce inconsistencies: collective judgments that fail to respect the interconnections between the issues. Secondly, not all methods of aggregation are conducive to truth-tracking: we would like the collective judgments to be more reliable than the individual ones. Finally, many methods of aggregation are manipulable by strategic voting: they may incentivize group members to misrepresent their individual judgments. We prove new impossibility or possibility theorems on all three challenges, identifying what it takes to produce collective judgments in a consistent, truth-tracking, and non-manipulable manner.