Insa Diop

The paradox of the supply elasticity of cotton in Mali

Unit root tests: Common pitfalls and best practices

Since the seminal paper by Granger and Newbold (1974) on spurious regressions, applied econometricians have become aware of the consequences of unit roots in empirical analysis with time series data. Yet one can still find many published papers with unit root tests implemented in an inappropriate way. The objective of this Technical Note is to highlight the common pitfalls and best practices when testing for unit roots. In addition to the theoretical discussion, we provide examples using price data from Kenya, Mali, Togo, and South Africa to illustrate the procedures we think are worth following.

Detecting threshold effects in price transmission

The analysis of price transmission plays a key role in understanding markets integration. This helps identify the nature of the relationship between geographically distant markets and cross-commodity price transmission, as well as the impact of liberalization policies and the identification of regions exposed to systemic shocks. This technical contributes to the debate between symmetric and asymmetric price transmission and proposes to present the traditional and new approaches for detecting threshold effects in price transmission while focusing on their advantages and limitations. There is no one-size-fits-all method to detect threshold effects in price transmission. Experts need to select a combination of elements (context of study, the economy under consideration, data availability…) to justify the relevancy of their choice. Beyond the presentation of the methods for detecting thresholds in price transmission, we perform an application in the case of the rice market in Senegal. The results support the evidence of an asymmetric price transmission between world and domestic prices in the short-run and a symmetric transmission in the long-run.