COMMODITY PRICES AND UNIT ROOT TESTS

Dabin Wang and William G. Tomek

No 19032, 2004 Conference, April 19-20, 2004, St. Louis, Missouri from NCR-134 Conference on Applied Commodity Price Analysis, Forecasting, and Market Risk Management

Abstract: Endogenous variables in structural models of agricultural commodity markets are typically treated as stationary. Yet, tests for unit roots have rather frequently implied that commodity prices are not stationary. This seeming inconsistency is investigated by focusing on alternative specifications of unit root tests. We apply various specifications to Illinois farm prices of corn, soybeans, barrows and gilts, and milk for the 1960 through 2002 time span. The preponderance of the evidence suggests that nominal prices do not have unit roots, but under certain specifications, the null hypothesis of a unit root cannot be rejected, particularly when the logarithms of prices are used. If the test specification does not account for a structural change that shifts the mean of the variable, the results are biased toward concluding that a unit root exists. In general, the evidence does not favor the existence of unit roots.

Keywords: Marketing (search for similar items in EconPapers)
Pages: 31
Date: 2004
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Citations: View citations in EconPapers (5)

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Persistent link: https://EconPapers.repec.org/RePEc:ags:ncrfou:19032

DOI: 10.22004/ag.econ.19032

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