This paper addresses the issue of numeric calculations related to the modelling of the probability of dying within a year among people aged 80 years and over. According to 'The Force of Mortality between the Ages of 80 and 120' (A.R. Thatcher, V. Kannisto, J.W. Vaupel, 1999), the applicability of the following models were analysed: Gompertz, Weibul, Heligman-Pollard, Kannisto, logistic, and Coale-Kisker. After appropriate modification of the formulas, the authors achieved a better goodness of fit for the logistic and the Coale-Kisker models than that reported in the aforementioned publication. A hyperbolic-tangent model was proposed - being an equivalent to the logistic model - and it provides a much better goodness of fit for the empirical data.