The formula suggested almost 50 years ago by Stoodley (cf. Reference 1) to allow for a reduction in rates of interest with the passage of time does not seem to have achieved the recognition which one might have expected, particularly in view of its easy application in practice. In his remarks summarising the discussion following his paper Stoodley conceded that the problem of choosing the parameters of the formula led to somewhat intractable equations, which could be solved only approximately by trial and error techniques. However, with modern calculation methods it is a trivial matter to solve this problem accurately and in §3 below we indicate one possible approach. On the other hand, the use of modern calculators considerably reduces the specific advantage of the original formula. Accordingly in §4 we consider the more general logistic curve (of which Stoodley's formula is a special case), which permits somewhat greater flexibility in modelling future trends.