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diff --git a/problem.tex b/problem.tex index f0eeb43..4835be4 100644 --- a/problem.tex +++ b/problem.tex @@ -131,15 +131,24 @@ etc.). The cost $c_i$ is the amount the subject deems sufficient to incentivize her participation in the study. Note that, in this setup, the feature vectors $x_i$ are public information that the experimenter can consult prior the experiment design. Moreover, though a subject may lie about her true cost $c_i$, she cannot lie about $x_i$ (\emph{i.e.}, all features are verifiable upon collection) or $y_i$ (\emph{i.e.}, she cannot falsify her measurement). %\subsection{D-Optimality Criterion} +<<<<<<< HEAD Ideally, motivated by the $D$-optimality criterion, we would like to design a mechanism that maximizes or approximates \eqref{dcrit} . Since \eqref{dcrit} may take arbitrarily small negative values, to define a meaningful approximation one would consider the (equivalent) maximization of $V(S) = \det\T{X_S}X_S$. %, for some strictly increasing, on-to function $f:\reals_+\to\reals_+$. However, the following lower bound implies that such an optimization goal cannot be attained under the constraints of truthfulness, budget feasibility, and individual rationality. \begin{lemma} For any $M>1$, there is no $M$-approximate, truthful, budget feasible, individionally rational mechanism for a budget feasible reverse auction with $V(S) = \det{\T{X_S}X_S}$. +======= +Ideally, motivated by the $D$-optimality criterion, we would like to design a mechanism that maximizes \eqref{dcrit} within a good approximation ratio. In what follows, we consider a slightly more general objective as follows: +\junk{ +As \eqref{dcrit} may take arbitrarily small negative values, to define a meaningful approximation one would consider the (equivalent) maximization of $V(S) = \det\T{X_S}X_S$. %, for some strictly increasing, on-to function $f:\reals_+\to\reals_+$. +However, the following lower bound implies that such an optimization goal cannot be attained under the constraints of truthfulness, budget feasibility, and individual rationality. +\begin{lemma} +For any $M>1$, there is no $M$-approximate, truthful, budget feasible, individually rational mechanism for a budget feasible reverse auction with value function $V(S) = \det{\T{X_S}X_S}$. +>>>>>>> c29302b25adf190f98019eb8ce5f79b10b66d54d \end{lemma} \begin{proof} \input{proof_of_lower_bound1} \end{proof} -This negative result motivates us to study a problem with a modified objective: +This negative result motivates us to study a problem with a modified objective:} \begin{center} \textsc{ExperimentalDesignProblem} (EDP) \begin{subequations} @@ -150,8 +159,11 @@ This negative result motivates us to study a problem with a modified objective: \end{subequations} \end{center} where $I_d\in \reals^{d\times d}$ is the identity matrix. One possible interpretation of \eqref{modified} is that, prior to the auction, the experimenter has free access to $d$ experiments whose features form an orthonormal basis in $\reals^d$. However, \eqref{modified} can also be motivated in the context of \emph{Bayesian experimental design} \cite{chaloner1995bayesian}. In short, the objective \eqref{modified} arises naturally when the experimenter retrieves the model $\beta$ through \emph{ridge regression}, rather than the linear regression \eqref{leastsquares} over the observed data; we explore this connection in Section~\ref{sec:bed}. +} -Note that maximizing \eqref{modified} is equivalent to maximizing \eqref{dcrit} in the full-information case. In particular, $\det(\T{X_S}X_S)> \det(\T{X_{S'}}X_{S'})$ iff $\det(I_d+\T{X_S}X_S)>\det(I_d+\T{X_{S'}}X_{S'})$. In addition, \eqref{edp}---and the equivalent problem with objective \eqref{dcrit}---are NP-hard; to see this, note that \textsc{Knapsack} reduces to EDP with dimension $d=1$ by mapping the weight of each item $w_i$ to an experiment with $x_i=w_i$. Nevertheless, \eqref{modified} is submodular, monotone and satifies $V(\emptyset) = 0$. +We present our results with this version of the objective function because it is simple and captures the versions +we will need later (including an arbitrary matrix in place of $I_d$ or a zero matrix which will correspond to ~\eqref{dcrit} ). +Note that maximizing \eqref{modified} is equivalent to maximizing \eqref{dcrit} in the full-information case. In particular, $\det(\T{X_S}X_S)> \det(\T{X_{S'}}X_{S'})$ iff $\det(I_d+\T{X_S}X_S)>\det(I_d+\T{X_{S'}}X_{S'})$. In addition, \eqref{edp}---and the equivalent problem with objective \eqref{dcrit}---are NP-hard; to see this, note that \textsc{Knapsack} reduces to EDP with dimension $d=1$ by mapping the weight of each item $w_i$ to an experiment with $x_i=w_i$. Nevertheless, \eqref{modified} is submodular, monotone and satisfies $V(\emptyset) = 0$. %, allowing us to use the extensive machinery for the optimization of submodular functions, as well as recent results in the % context of budget feasible auctions \cite{chen,singer-mechanisms}. |