Definition: The initial mortality rate (IMR) represents the mortality rate independently of the ageing process. Though there are different methods, the IMR is typically estimated from the mortality rate prior to its exponential increase with age, which can be obtained from the mortality curve (by looking at the baseline mortality prior to its exponential increase) or, for mice, by using t = 0.25 in the Gompertz equation.
Attribution: From senescence.info. Human Ageing Genomic Resources (HAGR). Estimating the Rate of Ageing. http://genomics.senescence.info/software/demographic.html