Documentation of deaths was poor during the Great Frost. Cemeteries provide fragmentary information, e.g., during February and March 1740, 47 children were buried in St. Catherine's parish. The normal death rate tripled in January and February 1740, and burials averaged out about 50% higher during the twenty-one-month crisis than for the years 1737-1739, according to Dickson. Summing up all his sources, Dickson suggests two estimates:1) that 38% of the Irish population died during the crisis, and2) that between 13-20% excess mortality occurred for 1740-1741. Based on contemporary accounts and burial parish records, famine-related deaths may have totaled 300,000-480,000 in Ireland, with rates highest in the south and east of the country. This was a proportionately greater toll than during the most severe years of the Great Famine .  That famine, however, was unique in "cause, scale and timing," persisting over several years. The majority of the deaths occurred amongst the lower classes, with the poorest third of the population gaining all their nutrition from potatoes alone, the crop which the blight affected. Therefore, when the potatoes blight occurred these people were left with practically no means of sustenance.
Question: How many years from 1740-1741 did the 13-20% excessive mortality rate last?
Answer: 1