A central question 1 of crowdsourcing is how to elicit expertise from agents. This is even more difficult when answers cannot be directly verified. A key challenge is that sophisticated agents may strategically withhold effort or information when they believe their payoff will be based upon comparison with other agents whose reports will likely omit this information due to lack of effort or expertise.Our work defines a natural model for this setting based on the assumption that more sophisticated agents know the beliefs of less sophisticated agents.We then provide a mechanism design framework for this setting. From this framework, we design several novel mechanisms, for both the single and multiple tasks settings, that (1) encourage agents to invest effort and provide their information honestly; (2) output a correct "hierarchy" of the information when agents are rational.
arXiv:1802.08312v2 [cs.GT] 22 May 2018 2 Mechanism Design ToolsWe use two key information theory ingredients in designing information elicitation mechanisms. The first ingredient is f -mutual information M I f (X; Y ) which measures the amount of information crossing two random variables X, Y . For example, if X is independent with Y -no information crosses X and Y , M I f (X; Y ) = 0. The second ingredient is proper scoring rule P S(x, p) which measures the accuracy of the prediction p even we only have one sample x of the outcome X.Both two ingredients have the information monotonicity property. If the information is measured by f -mutual information, any "data processing" on either of the random variables will decrease the amount of information crossing them. If the accuracy of a forecast is measured by a proper scoring rule, more information implies a more accurate forecast.
f -mutual informationwhere f (⋅) is a convex function and f (1) = 0. Two commonly used f -divergences are KL divergence and total variation distance. Now we start to introduce f -mutual information.Given two random variables X, Y , let U X,Y and V X,Y be two probability measures where U X,Y is the joint distribution of (X, Y ) and V is the product of the marginal distributions of X and Y . Formally, for every pair of (x, y),If U X,Y is very different with V X,Y , the mutual information between X and Y should be high since knowing X changes the belief for Y a lot. If U X,Y equals to V X,Y , the mutual information between X and Y should be zero since X is independent with Y . Intuitively, the "distance" between U X,Y and V X,Y represents the mutual information between them.Definition 2.1 (f -mutual information [20]). The f -mutual information between X and Y is defined aswhere D f is f -divergence. Definition 2.2 (Conditional f -mutual information [20]). Given three random variables X, Y, Z, we define M I f (X; Y Z) as z P r[Z = z]M I f (X; Y Z = z) where M I f (X; Y Z = z) ∶= M I f (X ′ ; Y ′ ) where P r[X ′ = x, Y ′ = y] = P r[X = x, Y = y Z = z].