Mortality and offtake rates, as other rates that will be described in Chap. 4, can represent two distinct parameters: a probability (say p) or a hazard rate (say h). Although definitions for p and h are often presented as preliminaries in statistical books [42–44], this is less common in tropical livestock literature, except in few specialized documents [45, 46]. For instance, both ILCA and CIRAD manuals, published in the 1990s, on research methods for tropical farming systems [10, 11] do not provide a clear distinction between p and h, while they represent different parameters. As an example, mortality rates for the four animals presented in Fig. 3.1 can be calculated by: $$ p=\frac{2}{4}=0.50 $$ $$ h=\frac{2}{\left(50+365+365+280\right)/365}=0.69/\mathrm{year} $$ In (3.1) the denominator is the initial number of animals and in (3.2) the time of presence of the animals in the considered period, referred to as time at risk in epidemiology.