Given an assembly of elements, the number of which decreases ultimately to zero, the<unk>  (also called the<unk>  is a certain number that characterizes the rate of reduction ("decay") of the assembly.  Specifically, if the individual<unk>  of an element of the assembly is the time elapsed between some reference time and the removal of that element from the assembly, the<unk>  is the arithmetic<unk>  of the individual<unk> s.
Typically, the notion of<unk>  is used in connection with exponential decay.  The remainder of this article confines itself to this particular decay pattern.
Mean lifetime
Incidence 
Prevalence
Average
Period prevalence
Dose Profile
Median
Recent changes