پاسخ های سیاست پولی بهینه به تغییرات نسبی قیمت

An optimizing model, with a flexible-price sector and a sticky-price sector, is presented to analyze the effects of relative-price changes on inflation fluctuations. The relative price of the flexible-price good represents a shift parameter of the New Keynesian Phillips curve. The optimal monetary policy is to target sticky-price inflation, rather than a broad inflation measure. Although stabilizing the relative price around its efficient value is one of the appropriate goals of the central bank, stabilizing sticky-price inflation is sufficient for achieving this goal. An optimal monetary policy for a small open economy is also discussed.

Relative prices—for example, of food and energy—, are often discussed in studies of inflation control for two reasons. First of all, relative prices are often used as measures of “supply shocks” in “Phillips curve” equations that seek to model the short run output-inflation trade-off.1 In the empirical literature on the Phillips curve, changes in the relative prices of food and energy are commonly used as a measure of supply shocks, which shift the short-run Phillips curve. Second, many authors have sought to identify a more persistent component of inflation, known as “core inflation”.2 For the conduct of monetary policy, core inflation is considered a more important indicator than broader inflation measures. In this literature, fluctuations in the prices of food and energy are regarded as a transitory component of overall movements in inflation, since they are thought to be caused mainly by temporary, sector-specific shocks. Based on this idea, it is a common practice to subtract the prices of food and energy from an aggregate inflation measure to calculate a measure of core inflation. However, it is not obvious how changes in relative prices affect aggregate inflation. Strictly speaking, a pure relative disturbance is a change in supply or demand conditions that leaves the appropriately defined aggregate production possibility frontier unchanged. In the absence of price stickiness, this shock should not change aggregate real output and the aggregate price level.3 It is also not obvious how relative-price changes are related to supply shocks. Large changes in relative prices are not necessarily caused by large supply shocks. Relative prices are affected by several factors other than supply shocks, such as demand shocks and elasticities of substitution among goods. These arguments suggest that the appropriate measures of supply shocks and core inflation should be based on a structural model which identifies the factors that affect relative-price changes and the persistent component of aggregate inflation. In this paper, we construct a two-sector dynamic general equilibrium model, with a flexible-price sector and a sticky-price sector. The model is a variant of optimizing models with nominal price stickiness, that have recently been used in the literature on inflation dynamics and monetary policy.4 Using this model we discuss the correct specification of the Phillips curve in the presence of sectoral shocks, and show in what way changes in the relative price of flexible-price good shift the short-run Phillips curve. We also show that the inflation in the sticky-price sector represents a persistent component of inflation, in the sense that it responds to smoothed expectations of the future output gaps and relative-price changes. Inflation in the sticky-price sector is therefore a good candidate for a measure of core inflation. Another important question is how a central bank should conduct monetary policy in the presence of sector-specific shocks that affect the efficient relative prices of the different types of goods. The central bank has a choice among several different possible measures of inflation and output gap, and it must identify which variables are the appropriate goal variables. Using an optimizing model has an important advantage; namely, it allows us to evaluate alternative monetary policies in a welfare-theoretic framework, and to analyze which variables should be stabilized in the optimal equilibrium. The paper shows that the optimal monetary policy is characterized as an inflation targeting regime, that incorporates the correctly chosen inflation measure. It is also found to be desirable to stabilize core inflation, rather than a broader measure of inflation, where core inflation is identified as an index of inflation in the sticky-price sector. The paper demonstrates that stabilizing the relative price of the flexible-price good around its time-varying optimal value is one of the appropriate goals for the central bank. However, the model implies that stabilizing core inflation is sufficient for keeping the relative price at its efficient value. We also address the issue on the output gap-inflation variability trade-off, which has been an important guiding principle in studies of monetary policy. Our model implies that there is a trade-off between stabilizing the aggregate output gap and aggregate inflation, but that there is no trade-off between stabilizing aggregate output gap and stabilizing core inflation. Thus, in this model, whether output gap-inflation variability trade-off exists or not depends on which measures of inflation and output gap the central bank chooses to stabilize. The organization of the paper is as follows. Section 2 presents a two sector dynamic sticky-price model, and studies the interaction between relative price and inflation determination. Section 3 derives the optimal monetary policy for the economy described in the model, and applies the model to an small open economy setting. Section 4 concludes.

In this paper, we addressed two important questions for a central bank under the existence of sector-specific supply shocks: the relationship between relative-price changes and inflation fluctuations, and the identification of appropriate goal variables for the central bank. We showed that the relative price of the flexible-price good represents a shift parameter of inflation in the sticky-price sector. This feature of the model is in agreement with the findings of the empirical literature on the Phillips curve that the relative price of food and energy is significant source of supply shocks. We also characterized the optimal monetary policy that maximizes the welfare of the representative household. The optimal monetary policy for the economy described in the model is a complete stabilization of the inflation in the sticky-price sector. This result implies that the central bank should target core inflation, defined as inflation in the sticky-price sector, rather than a broad inflation measure. Furthermore, the model predicts that stabilizing core inflation and stabilizing the aggregate output gap are consistent each other. The model also predicts that, although stabilizing the relative price around its efficient level is one of the appropriate goals of the central bank, stabilizing core inflation is sufficient for achieving this goal. The characterization of an optimal monetary policy would critically depend on the central bank's goal variables and propagation mechanism of business cycles. The appropriate goal variables, in turn, depend on the structure of the economy. It is therefore important to use an optimizing model to identify the appropriate goal variables of the central bank and its optimal policy regime.34