Incidence Density is no Density
Lucien Karhausen · Scandinavian Journal of Social Medicine · 1993
Miettinen’s “Incidence Density ” (1) corresponds to the average number of cases per unit person-time under risk in a given time interval. Expressions such as incidence (or velocity) which represent the gra-dient of the curve obtained by plotting a continuous and differentiable mathematical function, can be ex-pressed in two ways, average incidence and instan-taneous or point incidence. Incidentally, the same distinction applies t o density. The expression “Incidence Density ” may be a source of confusion, since i t is contrary to customary’ mathematical usage according to which instantane-ous, not average incidence, should be called inci-dence densiiy. Yet, this expression is more than a mere mis-