Spatiotemporal factor models for functional data with application to population map forecast

With the proliferation of mobile devices, an increasing amount of population data is being collected, and there is growing demand to use the large-scale, multidimensional data in real-world situations. We introduced functional data analysis (FDA) into the problem of predicting the hourly population of different districts of Tokyo. FDA is a methodology that treats and analyzes longitudinal data as curves, which reduces the number of parameters and makes it easier to handle high-dimensional data. Specifically, by assuming a Gaussian process, we avoided the large covariance matrix parameters of the multivariate normal distribution. In addition, the data were time and spatially dependent between districts. To capture these characteristics, a Bayesian factor model was introduced, which modeled the time series of a small number of common factors and expressed the spatial structure in terms of factor loading matrices. Furthermore, the factor loading matrices were made identifiable and sparse to ensure the interpretability of the model. We also proposed a method for selecting factors using the Bayesian shrinkage method. We studied the forecast accuracy and interpretability of the proposed method through numerical experiments and data analysis. We found that the flexibility of our proposed method could be extended to reflect further time series features, which contributed to the accuracy.