Learning and Model Validation

This paper studies the following problem. An agent takes actions based on a possibly misspecified model. The agent is 'large', in the sense that his actions influence the model he is trying to learn about. The agent is aware of potential model misspecification and tries to detect it, in real-time, using an econometric specification test. If his model fails the test, he formulates a new better-fitting model. If his model passes the test, he uses it to formulate and implement a policy based on the provisional assumption that the current model is correctly specified, and will not change in the future. We claim that this testing and model validation process is an accurate description of most macroeconomic policy problems. Unfortunately, the dynamics produced by this process are not well understood. We make progress on this problem by relating it to a problem that is well understood. In particular, we relate it to the dynamics of constant-gain stochastic approximation algorithms. This enables us to appeal to well known results from the large deviations literature to help us understand the dynamics of testing and model revision. We show that as the agent applies an increasingly stringent specification test, the large deviation properties of the discrete model validation dynamics converge to those of the continuous learning dynamics. This sheds new light on the recent constant-gain learning literature.Learning, Validation, Relative Entropy, Large Deviation

Three implications of learning behaviour for price processes.

no abstract availableConsumers' preferences; Economics -- Psychological aspects;

The virtues and vices of equilibrium and the future of financial economics

The use of equilibrium models in economics springs from the desire for parsimonious models of economic phenomena that take human reasoning into account. This approach has been the cornerstone of modern economic theory. We explain why this is so, extolling the virtues of equilibrium theory; then we present a critique and describe why this approach is inherently limited, and why economics needs to move in new directions if it is to continue to make progress. We stress that this shouldn't be a question of dogma, but should be resolved empirically. There are situations where equilibrium models provide useful predictions and there are situations where they can never provide useful predictions. There are also many situations where the jury is still out, i.e., where so far they fail to provide a good description of the world, but where proper extensions might change this. Our goal is to convince the skeptics that equilibrium models can be useful, but also to make traditional economists more aware of the limitations of equilibrium models. We sketch some alternative approaches and discuss why they should play an important role in future research in economics.Comment: 68 pages, one figur

Are Inflation Expectations Rational?

Simple econometric tests reported in the literature consistently report what appears to be a bias in inflation expectations. These results are commonly interpreted as constituting evidence overturning the hypothesis of rational expectations. In this paper, we investigate the validity of such an interpretation. The main tool utilized in our investigation is a computational dynamic general equilibrium model capable of generating aggregate behavior similar to the data along a number of dimensions. By construction, the model embedded the assumption of rational expectations. Standard regressions run on equilibrium realizations of inflation and inflation expectations nevertheless reveal an apparent bias in inflation expectations. In these simulations, the null hypothesis of rational expectations is incorrectly rejected in a large percentage of cases; a result that casts some doubt on conventional interpretations of the evidence.

An Interview with Thomas J. Sargent

The rational expectations hypothesis swept through macroeconomics during the 1970’s and permanently altered the landscape. It remains the prevailing paradigm in macroeconomics, and rational expectations is routinely used as the standard solution concept in both theoretical and applied macroeconomic modelling. The rational expectations hypothesis was initially formulated by John F. Muth Jr. in the early 1960s. Together with Robert Lucas Jr., Thomas (Tom) Sargent pioneered the rational expectations revolution in macroeconomics in the 1970s. We interviewed Tom Sargent for Macroeconomic Dynamics .

Are Inflation Expectations Rational?

Several recent papers report evidence of an apparent statistical bias in inflation expectations and interpret these findings as overturning the rational expectations hypothesis. In this paper, we investigate the validity of such an interpretation. We present a computational dynamic general equilibrium model capable of generating aggregate behavior similar to the data along several dimensions. By construction, model agents form "rational" expectations. We run a standard regression on equilibrium realizations of inflation and inflation expectations over sample periods corresponding to those tests performed on actual data and find evidence of an apparent bias in inflation expectations. Our experiments suggest that this incorrect inference is largely the product of a small sample problem, exacerbated by short-run learning dynamics in response to infrequent shifts in monetary policy regimes.Regime changes; Learning dynamics; Monte Carlo exp eriments; Sample size.

Market efficiency today

This CFS Working Paper has been presented at the CFSsymposium "Market Efficiency Today" held in Frankfurt/Main on October 6, 2005. In 2004 the Center for Financial Studies (CFS) in cooperation with the Johann Wolfgang Goethe University, Frankfurt/Main established an international academic prize, which is to be known as The Deutsche Bank Prize in Financial Economics. The prize will honor an internationally renowned researcher who has excelled through influential contributions to research in the fields of finance and money and macroeconomics, and whose work has lead to practice and policy-relevant results. The Deutsche Bank Prize in Financial Economics has been awarded for the first time in October 2005. The prize, sponsored by the Stiftungsfonds Deutsche Bank im Stifterverband für die Deutsche Wissenschaft, carries a cash award of € 50,000. The prize will be awarded every two years and the prize holder will be appointed a "Distinguished Fellow" of the CFS. The role of media partner for the Deutsche Bank Prize in Financial Economics is to be filled by the internationally renowned publication, The Economist and the Handelsblatt, the leading German-language financial and business newspaper

A Behavioral Approach to Learning in Economics - Towards an Economic Theory of Contingent Learning

In economics, adjustment of behavior has traditionally been treated as a "black box." Recent approaches that focus on learning behavior try to model, test, and simulate specific adjustment mechanisms in specific environments (mostly in games). Results often critically depend on distinctive assumptions, and are not easy to generalize. This paper proposes a different approach that aims to allow for more general conclusions in a methodologically more compatible way. It is argued that the introduction of the main determinants of learning behavior as situational restrictions into the standard economic model may be a fruitful way to capture some important aspects of human behavior that have often been omitted in economic theory. Based on a simple model of learning behavior (learning loop), robust findings from psychology are used to explain behavior adjustment, and to identify its determinants (contingent learning). An integrative methodology is proposed where the "black box" is not opened, but instead the factors that determine what happens inside, and the limits imposed by theses factors can be analyzed and used for model building. The paper concludes with testable hypotheses about learning behavior in the context of economics.microeconomics, game theory, learning theory, experiments

A “Local” Model of the Firm: Sticky prices and the Phillips Curve

Assume a firm concerns itself exclusively with local shocks (copious citations including Lucas 1972 and Bomhoff 1983 validate that this type of assumption may be reasonable). Changes in a firm's production policy should occur when the actual demand in a period Dt suggests that the underlying demand function has shifted from expected demand E(Dt). Since firms face uncertainty, this is non-trivial and they must find a way to determining (given information from a single, current period) whether or not the underlying demand has changed or whether the firm has simply obtained a draw from its expected demand distribution. In a simplified model, a firm can use a concept similar to a Statistical Hypothesis Test on E(Dt) = Dt to come to this conclusion. Rather than select an arbitrary confidence threshold (alpha), a firm can reverse the process and use the "marginal" alpha (where the hypothesis is just rejected or accepted) as its confidence that the mean has changed, allowing it to update its expectations to E(Dt+1) = (1-a) * E(Dt) + a * (Dt) and price accordingly. By weighting new demand information using this "confidence factor," the model introduces significant and persistent rigidity around NAIRU/equilibrium. This model is also powerful because it explains the qualified success of threshold like behaivor in classical "menu cost" theories (as the threshold reflects the classic hypothesis test strategy), behavior similar to a learning model (via the weighted introduction of new data) and seeming information lags (via the low confidence in new information immediately after shifts), among others.phillips curve