Potential output is of vital importance in macroeconomic policy making, despite imperfections in its estimation.
In order to have an effective monetary and fiscal policy, policy makers need to gauge the level of economic activity in the economy and whether this level is consistent with the potential level. In economic terms, policy makers look at the real output and its deviation from potential output, called the output gap. Potential output – the trend growth in the productive capacity of an economy – is an estimate of the level of GDP attainable when the economy is operating at a high rate of resource use. This is not a technical ceiling on the maximum level of output attainable. Rather, it is an estimate of maximum sustainable output – output that can be sustained in the long run without leading to macroeconomic instability.
While this may seem like a purely academic and statistical exercise, in reality, understanding and proper estimation of potential output has grave consequences for the economy. If actual output is lower than potential output, that is, if the output gap is negative, then the economy is performing below its potential – resources and capacity are underutilized and unemployment is higher than what it should be. On the other hand if the output gap is positive (actual output is higher than potential output), the economy is overheated, demand exceeds supply and inflationary pressures on the economy is high. As is self-evident, neither state is desirable. The manifestation of the output gap is usually through inflation in the economy. A positive output gap results in higher inflation and a negative output gap results in deflation.
For policy makers, therefore, understanding potential output and the output gap is of crucial importance. Negative output gap should ideally be followed by an expansionary fiscal and monetary policy, so as to increase spending and demand in the economy, which will result in actual output converging towards potential output. A contractionary monetary and fiscal policy is required in the case of a positive output gap to reduce the demand in the economy and to provide liquidity to the suppliers to increase their production.
Many central bankers around the world indeed use the concept of potential output in determining the rate of interest. In deciding the policy rate, central bankers use a popular rule of thumb called the Taylor rule, which reduces the complexities in choosing the interest rate to a formula that incorporates the difference between the actual and targeted inflation rate and the difference between the actual and potential GDP.
As can be seen from the graph, real GDP has exceeded potential GDP during the boom years in the late 1990s and has significantly fallen below the potential GDP after the recession off 2007. It can also be seen that inflation reacts to the output gap. Inflation is above the targeted rate of 2% when output gap is positive and vice versa.
Estimating Potential Output
Despite its overwhelming importance to policy making, there seems to be no consensus amongst economists regarding the best method to estimate potential output. Different countries and organizations use different methods based on country specific circumstances. However, no method has been able to provide consistently robust estimates and each method has its own set of lacunae.
The various methods of estimating potential GDP can be broadly classified into two categories: the production function approach and the statistical approach. The first approach, followed by the Congressional Budget Office, USA, relates the level of output to level of technology and factor inputs, namely capital and labour. Potential Output, thus, would be the output if both labour and capital are fully utilized in an efficient manner. This manner would also require certain assumptions regarding the specific form of the production to be made. Usually, a constant returns to scale production function, such as the Cobb-Douglas production function, is used.
However, for emerging market economies, where reliable data on labour and capital is unavailable, time-series statistical techniques have become quite popular. A widely used approach in the Indian context is the Hodrick-Prescott filter, which decomposes the actual real GDP into two components – a trend and a cyclical component – and potential output is proxied by the trend component. In other words, the GDP growth rate has an underlying structural component (trend) and another component that is seemingly random due to natural variations in the business cycles and external demand and supply shocks (cyclical). The purpose of the statistical tools is to remove the cyclical part and project the long run potential GDP based on the trend growth rate.
The graph below shows the estimates of output gap for India using various statistical techniques. While there are differences between the different techniques, broad generalizations can be derived: the economy was overheated for a prolonged period between 2005 and 2012 and has been in slack ever since.
Irrespective of which method is used, it is important to understand the shortcomings in these approaches. However, the presence of short-comings should not be a reason to undermine the immense importance of the concept of potential output in determining macroeconomic policies.
Anupam Manur is a policy analyst at Takshashila Institution. He can be found on twitter @anupammanur
 Specifically, it is: it = i* + α (πt – π*) + β (yt – y*), where it is the policy rate; πt and π* are the actual and targeted inflation rates, respectively; yt and yt* are actual and potential output, respectively; and i* is the federal funds rate consistent with on-target inflation and output.