Demand relations of oilseed products in South Africa

[摘要] English: In this study demand relations for primary oilseeds in South Africa is estimated and interpreted with the use of econometric models. Two different models, namely the Linear Approximate Almost Ideal Demand System (LA/AIDS) and the two-step Error Correction Model (ECM), were applied to annual oilseed data for the years 1971-2002. The F ratio test for separability failed to reject the null hyp othesis of weak separability in most cases, indicating that sunflower seed, soybeans, groundnuts and cotton could be included in the same system and modeled together. The Hausman test for exogeneity was conducted and proved that the expenditure variable included in the estimated equations is indeed exogenous. The exogeneity of the expenditure variable provides assurance that the Restricted Seemingly Unrelated Regression (RSUR) method of estimation will provide efficient parameter estimates. Both the short run models are estimated in differenced form, from where the parameter estimates obtained were used to calculate compensated, uncompensated and expenditure elasticities of demand. The compensated own price elasticity of soybeans is the largest in absolute terms, with coefficients ranging from -0.579 in the LA/AIDS to -0.666 in the ECM. Seed cotton has the second largest compensated own price elasticity with -0.399 and -0.542 respectively in the two models. The compensated cross product elasticities indicate a predominantly substituting relationship between these oilseeds, even though not all of them are significant. According to the calculated uncompensated own price elasticities, seed cotton is the most price responsive i.e. (-0.745) in the ECM and soybeans (-0.617), in the LA/AIDS. According to the expenditure elasticities sunflower seed (1.105) and cotton (1.064) can be regarded as luxury oilseeds in South Africa. Soybeans, with expenditure elasticities of between 0.454 and 0.493 in the two respective models, can be regarded as a normal good. Groundnuts can also be regarded as a luxury commodity even though it has an expenditure elasticity of just below one. The fact that the compensated own price elasticity of groundnuts is smaller in absolute terms than the expenditure elasticity is also an indication of a luxury product, as proved by Hicks and Juréen (1962).

[发布机构] University of the Free State