TESTS FOR SEASONAL DIFFERENCING WITH AN UNKNOWN BREAK‐POINT

Summary Some Lagrange multiplier tests for seasonal differencing are proposed; their main objective is to avoid over‐differencing due to structural change. The null hypothesis is either the presence of both regular and seasonal unit roots or the presence of a seasonal unit root. Alternative hypotheses allow for stationarity around a possible structural change where the break‐point is unknown. The location of the structural change is estimated using the proposed procedures, the asymptotic distribution of the test statistics under the null hypothesis is derived and some useful percentiles are tabulated. An illustrative example based on the Canadian Consumer Price Index is presented.