RDP 9306: Inventories and the Business Cycle 1. Introduction

On average, over the past 30 years inventory investment has accounted for just 0.7 per cent of the level of Gross Domestic Product (GDP). In contrast, on average, quarterly changes in the level of inventory investment have accounted for 59 per cent of the change in GDP.^[1] In recessions, inventory investment has played an even more important role, often accounting for over 100 per cent of the fall in output. Clearly, while the accumulation of inventories has little impact on long-run economic growth, it is an extremely important part of the business cycle. This importance has led to a considerable research effort by both policy makers and business-cycle theorists into the behaviour of inventories. This research has failed to arrive at a consensus. Paralleling the ongoing debate concerning the source of shocks generating the business cycle, there are basically two broad schools of thought. One school argues that the inventory cycle is driven by shocks to demand while the other argues that it is driven by shocks to supply.

In this paper we examine the role of inventory investment in the Australian business cycle and use a previously unexplored dataset to examine the sources of fluctuations in inventories. The data are from the ACCI/Westpac Survey of Industrial Trends and consist of firms' expected and actual experience with inventories, costs and demand. By providing a direct measure of expectations, the survey results allow us to distinguish between expected and unexpected changes in these variables. Our results suggest that demand factors are of critical importance in driving the inventory cycle. In addition, they suggest that an increased ability of firms to manage their inventories has meant that unexpected changes in demand no longer generate the same pronounced inventory cycle that they once did.^[2] This conclusion is supported by the macro-economic data.

The demand side models of inventory investment were pioneered by the work of Metzler (1941) who argued that desired inventories were proportional to expected sales. This link between expected sales and inventories reflects two factors. The first is that marginal cost increases with output and the second is that firms hold inventories to reduce the probability of being unable to meet unexpectedly high demand. With increasing marginal costs, firms can minimise production costs by keeping a constant level of production, equal to expected sales. With production decisions being made before demand is known, a positive unanticipated shock to demand is met, in the first instance, by running down inventories. This fall in inventories, however, increases the probability of running out of stock and so leads to some additional increase in output in the subsequent period to rebuild stocks. A stock cycle is the result; inventory investment is negative in the first period and then positive in the second period.

In contrast, the supply side models focus on cost shocks. These models are based on the premise that the principal type of shock generating the business cycle is a shock to the production function. When a favourable but temporary productivity shock occurs, it reduces costs and induces the firm to increase output while costs are temporarily low. This extra output is stored as inventories and sold when output is temporarily low as the result of a bad productivity shock. Work by West (1989) has provided support for the cost-based models suggesting that cost shocks are the primary source of variation in inventories. This result mirrors the results of work by Prescott (1986) who estimated that cost shocks have accounted for more than 75 per cent of the variation in GDP.

In recent years, these supply side models have received increased attention. This interest primarily reflects two stylised facts. The first is that the variance of production exceeds the variance of sales. The second is that inventory movements are pro-cyclical.^[3] The first of these ‘facts’ is often interpreted as evidence against production smoothing and demand-based models of the inventory cycle. If firms are attempting to smooth production in the face of varying demand, why does the variance of production exceed the variance of sales? Proponents of cost-based theories argue that this greater variance reflects the high variance of productivity shocks relative to the variance of demand shocks. The positive correlation between inventory investment and output also suggests support for the cost-shock model. If costs are temporarily low, both output and inventories should increase, generating a positive correlation between the two variables. The demand-production smoothing based theories predict a negative correlation.

Given the role of inventory cycle in determining the amplitude of the output cycle it is important to understand the factors driving inventory investment. This is the task of the remainder of this paper. We begin in Section 2 by presenting a simple model of inventory behaviour that provides a framework for discussing the impact of various types of shocks on inventories and output. In the following section, we use macro-economic data to present some basic facts about inventory investment in Australia. We examine the contribution of inventory accumulation to the business cycle, the relationship between the variance of sales and the variance of production and the correlation between changes in inventories and changes in sales. In Section 4, we turn to the survey data to analyse the relationship between changes in demand and costs on the one hand, and expected and unexpected changes in inventories of finished goods and raw materials on the other. Finally, in Section 5 we summarise and conclude.

Footnotes

This figure is obtained by dividing the average absolute change in inventory investment by the average absolute change in Gross Domestic Product. [1]

It is important to distinguish between inventories and inventory investment. In this paper the term inventories is used to refer to the level of stocks while inventory investment is used to describe the change in the level of stocks. [2]

For evidence on the relationship between the variance of production and the variance of sales see Blanchard (1983), Blinder (1986), West (1988) and Blinder and Maccini (1991). For evidence on the pro-cyclical nature of inventory investment see Summers (1981), Blanchard and Fischer (1989) and West (1989). [3]