Structural analysis of the abortion time series reported in the ISTAT summaries

After the approval of the law on voluntary abortion in Italy, the Italian health care system started to practice voluntary abortion before the third month of pregnancy. Since 1980, the Italian Institute of Statistics (ISTAT) began to collect data on the abortion frequency per month and per administrative local areas. Although a preliminary analysis of the data showed that, after an initial increase of number of abortions, the number progressively lowered over years, there is no insight on the existence of periodicity in the time series and on the local effects related to the regional habits and social environments. The aim of our study is therefore to extract local trends and periodicity from the data collected by ISTAT, by combining a “structural model” of the time series and Bayesian Statistics. This paper describes the built stochastic model, its Bayesian estimation through a Markov Chain Monte Carlo approach on the Italian abortion data. Thanks to this analysis, it is possible to note that, differently from the original time series, the trend component is very regular and shows clearly how after an initial period of increase, after June-July 1983 the voluntary abortion trend decreases constantly until the end of the study. The periodic component shows an astonishing regularity too, suggesting that the Italian people have a seasonal preference for voluntary abortion.