Generalized Space Time Autoregressive (GSTAR) Modeling in Predicting the Price of Bird’s Eye Chili in East Java, West Java, and Central Java

Abstract

Bird’s eye chili is a commodity that can impact Indonesia’s economy. In 2022, Indonesia's bird’s eye chili production increased to 1,544,441 tons. The three provinces with the highest bird’s eye chili production are East Java, Central Java, and West Java. Over time, the price of bird’s eye chili in Indonesia always fluctuates. To address the uncertainty in bird’s eye chili prices, an appropriate analysis method is needed to predict the price of bird’s eye chili in order to help the government to formulate policies that are right on target, namely using the Generalized Space Time Autoregressive (GSTAR) method in the provinces of East Java, Central Java, and West Java with uniform location weighting, distance inverse, and cross-correlation normalization. The research data includes bird’s eye chili commodity prices in the three provinces in Rupiah with a daily period from February 2023 to November 2023 obtained from the National Food Agency Institute. Based on the analysis, it can be concluded that the best model to forecast the price of bird’s eye chili in the Provinces of East Java, Central Java, and West Java is GSTAR(21)I(1) after 1 differencing time, using uniform location weights. The advantages of this model can be seen in its conformity with the assumption of white noise, as indicated by the MAPE value of 2.018% in training data and 2.064% in testing data.