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The major problems for any ARIMA (*) user are (1) what about Black Swans and (2) what if the present does not resemble the recent past and (3) what happens when the errors are NOT independent and identically distributed - usually taken to be sampled from a normal distribution with a mean of zero. For example, ARIMA would suggest if dinosaurs dominated almost every niche on Planet Earth for 130 million years, they would likely continue to do so. Then an unpredictable event happens - a very large rock moving very quickly bashes into the Yucatan. No more dinosaurs. One of the Blackest of Black Swans. Unless you were a heretofore insignificant mammal. 

An example more relevant to Portugal: as far as I am aware Portugal has hosted 12 Roman Catholic saints: John of God (São João de Deus; 1495-1550 was born in Portugal but spent most of his life in Spain; he founded the Brothers Hospitallers of St. John of God - today more than 1500 Brothers care for the sick in over 100 hospitals with almost 13,000 beds); Saint Amador (or Amator about whom I can discover little); Anthony of Lisbon, O.F.M. (1195-1231; the patron saint of Portugal among other things); Saint John de Brito, S.J. (born 1647; martyred in India 1693); Saint Felix the Hermit (9th century); Saint Fructuosus of Braga (early 7th century); Saint Irene of Tomar (635? to 653?); Saint Martin of Braga (520? - 580?); Saint Ovidius (born in Sicily and sent to Braga as a bishop in 95; martyred in 135); the very singular Dom Nuno Álvares Pereira, O.Carm (1360-1431; his tomb was lost in the 1755 earthquake, canonized in 2009); Saint Peter of Rates (also known as Saint Peter of Braga where he was bishop from 45 until his martyrdom in 60); Saint Theotonius (1088 - 1166; founded the Monastery of the Holy Cross in Coimbra where Saint Anthony worked for a time. The original Romanesque buildings were replaced in the early 16th century - see the Wikipedia article on the Monastery of Santa Cruz at Coimbra.

If unpredictable but beneficial events can be called Silver Swans one might run a time series analysis and suggest that Portugal (about 85% Roman Catholic) is overdue for a saint.

* ARIMA = autoregressive integrated moving average - one way of analyzing time series data to either understand the past or predict the future