THE FORECASTING OF MONTHLY INFLATION IN YOGYAKARTA CITY USES AN EXPONENTIAL SMOOTHING-STATE SPACE MODEL

: The purpose of this research is to predict monthly inflation in the city of Yogyakarta with a simple and easy forecasting model that has high accuracy. The model used is the exponential smoothing-state space or known as the error, trend, and seasonal (ETS) model. This model does not have statistical assumptions, it is easy to analyze using R-package statistics which is an open-source program. This ETS model is built with a combination of trend and non-trend, seasonal and non-seasonal models as well as an additive or multiplicative errors. The monthly inflation data used in this research is secondary data obtained from the Central Bureau of Statistics (BPS) for the city of Yogyakarta from January 2015 to December 2021 with a total of 84 data. The results of this research obtained that the most suitable ETS model for predicting monthly inflation in the city of Yogyakarta is the ETS model (A, N, A). The ETS model (A, N, A) means that the error is additive (A), does not contain a trend (N) and seasonality is additive, so it is written as ETS (A, N, A). The ETS model (A, N, A) obtained in this research has an Akaike information criteria (AIC) value of 145.1996 with an RMSE forecasting accuracy value of 0.2166014 and a MAPE of 127.1662. The results of the forecasting for the next three periods show that the monthly inflation value of Yogyakarta is quite stable, there is an increase and decrease or it fluctuates slightly and is still below 10%.