... estimation and testing and describe enhanced routines that address HAC standard errors, weak instruments, LIML and k-class estimation, tests for endogeneity and RESET and autocorrelation tests for IV estimates.

Abstract: Since the oil crises of the 1970s there has been strong interest in the question of how oil production shortfalls caused by wars and other exogenous political events in OPEC countries affect oil prices, U.S. real GDP growth and U.S. CPI inflation. This study focuses on the modern OPEC period since 1973. The results differ from the conventional wisdom along a number of dimensions. First, it is shown that under reasonable assumptions the timing, magnitude and even the sign of exogenous oil supply shocks may differ greatly from current state-of-the-art estimates. Second, the common view that the case for the exogeneity of at least the major oil price shocks is strong is supported by the data for the 1980/81 and 1990/91 oil price shocks, but not for other oil price shocks. Notably, statistical measures of the net oil price increase relative to the recent past do not represent the exogenous component of oil prices. In fact, only a small fraction of the observed oil price increases during crisis periods can be attributed to exogenous oil production disruptions. Third, compared to previous indirect estimates of the effects of exogenous supply disruptions on real GDP growth that treated major oil price increases as exogenous, the direct estimates obtained in this paper suggest a sharp drop after five quarters rather than an immediate and sustained reduction in economic growth for a year. They also suggest a spike in CPI inflation three quarters after the exogenous oil supply shock rather than a sustained increase in inflation, as is sometimes conjectured. Finally, the results of this paper put into perspective the importance of exogenous oil production shortfalls in the Middle East. It is shown that exogenous oil supply shocks made remarkably little difference overall for the evolution of U.S. real GDP growth and CPI inflation since the 1970s, although they did matter for some historical episodes. Key Words: Oil shock; war; counterfactual; oil supply; exogeneity; weak instruments. JEL: E32, C32.

Large fluctuations in energy prices have been a distinguishing characteristic of the U.S. economy since the 1970s. Turmoil in the Middle East, rising energy prices in the U.S. and evidence of global warming recently have reignited interest in the link between energy prices and economic performance. This paper addresses a number of the key issues in this debate: What are energy price shocks and where do they come from? How responsive is energy demand to changes in energy prices? How do consumers ’ expenditure patterns evolve in response to energy price shocks? How do energy price shocks affect real output, inflation, stock markets and the balance-of-payments? Why do energy price increases seem to cause recessions, but energy price decreases do not? Why has there been a surge in gasoline prices in recent years? Why has this new energy price shock not caused a recession so far? Have the effects of energy price shocks waned since the 1980s and, if so, why? As the paper demonstrates, it is critical to account for the endogeneity of energy prices and to differentiate between the effects of demand and supply shocks in energy markets, when answering these questions.

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a b s t r a c t This paper is the first to use the method of instrumental variables (IV) to estimate the impact of obesity on medical costs in order to address the endogeneity of weight and to reduce the bias from reporting error in weight. Models are estimated using restricted-use data from the Medical Expenditure Panel Survey for 2000-2005. The IV model, which exploits genetic variation in weight as a natural experiment, yields estimates of the impact of obesity on medical costs that are considerably higher than the estimates reported in the previous literature. For example, obesity is associated with $656 higher annual medical care costs, but the IV results indicate that obesity raises annual medical costs by $2741 (in 2005 dollars). These results imply that the previous literature has underestimated the medical costs of obesity, resulting in underestimates of the economic rationale for government intervention to reduce obesity-related externalities.

University (Corporate Law Seminar) for helpful comments on a previous draft of this paper.

Abstract—We estimate the aggregate long-run elasticity of substitution between more educated workers and less educated workers (the slope of the inverse demand curve for more relative to less educated workers) at the U.S. state level. Our data come from the (five) 1950–1990 decennial censuses. Our empirical approach allows for state and time fixed effects and relies on time- and state-dependent child labor and compulsory school attendance laws as instruments for (endogenous) changes in the relative supply of more educated workers. We find the aggregate long-run elasticity of substitution between more and less educated workers to be around 1.5. I.

This paper considers tests of the parameter on an endogenous variable in an instrumental variables regression model. The focus is on determining tests that have some optimal power properties. We start by considering a model with normally distributed errors and known error covariance matrix. We consider tests that are similar and satisfy a natural rotational invariance condition. We determine a two-sided power envelope for invariant similar tests. This allows us to assess and compare the power properties of tests such as the conditional likelihood ratio (CLR), the Lagrange multiplier, and the Anderson–Rubin tests. We find that the CLR test is quite close to being uniformly most powerful invariant among a class of two-sided tests. The finite-sample results of the paper are extended to the case of unknown error covariance matrix and possibly nonnormal errors via weak instrument asymptotics. Strong instrument asymptotic results also are provided because we seek tests that perform well under both weak and strong instruments.

New goods play a central role in many trade and growth models. We use detailed trade and firmlevel data from a large developing economy—India—to investigate the relationship between declines in trade costs, the imports of intermediate inputs and domestic firm product scope. We estimate substantial static gains from trade through access to new imported inputs. Accounting for new imported varieties lowers the import price index for intermediate goods on average by an additional 4.7 percent per year relative to conventional gains through lower prices of existing imports. Moreover, we find that lower input tariffs account on average for 31 percent of the new products introduced by domestic firms, which implies potentially large dynamic gains from trade. This expansion in firms ' product scope is driven to a large extent by international trade increasing access of firms to new input varieties rather than by simply making existing imported inputs cheaper. Hence, our findings suggest that an important consequence of the input tariff liberalization was to relax technological constraints through firms ’ access to new imported inputs that were unavailable prior to the liberalization.

This paper analyzes the conditions under which consistent estimation can be achieved in instrumental variables (IV) regression when the available instruments are weak, in the local-to-zero sense of Staiger and Stock (1997) and using the many-instrument framework of Morimune (1983) and Bekker (1994). Our analysis of an extended k-class of estimators that includes Jackknife IV (JIV E) establishes that consistent estimation depends importantly on the relative magnitudes of rn, the growth rate of the concentration parameter, and Kn, the number of instruments. In particular, LIML and JIV E are consistent when Kn rn → 0, while two-stage least squares is consistent only if Kn rn → 0, as n→∞. We argue that the use of many instruments may be beneficial for estimation, as the resulting concentration parameter growth may allow consistent estimation, in certain cases. JEL classification: C13, C31.