Decreasing Marginal Value of Information Under Symmetric Loss

We investigate conditions under which the value of additional information is diminishing in a setting where the decision maker has access to multiple, identically-distributed, information signals. The signals are assumed to be independent conditional on an unknown payoff-relevant parameter. The decision maker minimizes a quadratic loss function. Quadratic losses arise in quality control, scoring rules, and other applications. We characterize two concepts of diminishing marginal value of information. The first is an ex ante concept, before any information is observed, and the second is an ex post concept, after observation of previous information signals. The former concept is useful for ex ante information acquisition decisions and the latter for sequential information acquisition.