Forecasting the Human Development Index of Toba Regency Using Holt’s Double Exponential Smoothing

The Human Development Index (HDI) of Toba Regency has generally shown a long-term upward trend, although a substantial change occurred during the observation period. Reliable forecasting is required to support regional planning, resource allocation, and policy evaluation; however, studies applying Holt’s two-parameter Double Exponential Smoothing to long-term HDI data at the regency level remain limited. Objective: This study aimed to model the historical pattern, evaluate forecasting accuracy, and project the HDI of Toba Regency for 2026–2030 using Holt’s Double Exponential Smoothing. Method: This quantitative time-series forecasting study used 22 annual secondary observations of Toba Regency’s HDI covering 2004–2025, obtained from Badan Pusat Statistik. All annual observations within the study period were included. The level and trend smoothing parameters were determined through a trial-and-error procedure based on the smallest forecasting error. Model performance was evaluated using Mean Absolute Deviation (MAD), Root Mean Squared Error (RMSE), and Mean Absolute Percentage Error (MAPE). Findings: The optimal parameters were for level smoothing and for trend smoothing. The model produced a MAD of 0.57, an RMSE of 1.34, and a MAPE of 0.77%, indicating a low in-sample forecasting error. The HDI was projected to increase from 79.60 in 2026 to 80.04 in 2027, 80.49 in 2028, 80.93 in 2029, and 81.37 in 2030. The forecast indicates a transition from the high to the very high HDI category beginning in 2027. Implications: These projections can support the Toba Regency Government in establishing development priorities and evaluating policies related to health, education, and living standards, while remaining subject to external socioeconomic and policy changes. Originality: This study provides a specific regency-level application of Holt’s separate level-and-trend smoothing parameters to a 22-year HDI series and identifies the projected timing of Toba Regency’s transition into the very high HDI category.