= Biostatistics
<Biostatistics> develops and applies <statistical inference> to biological and health questions. Its problems include designing experiments, comparing <treatments>, measuring biological variation and analyzing survival or repeated observations. The experimental unit must match the independent unit of information: multiple measurements of one organism need not substitute for measurements of multiple organisms.
Suppose a randomized study observes an event in 10 of 100 treated participants and 20 of 100 controls. The estimated <risk difference> is $0.10-0.20=-0.10$, while the estimated <relative risk> is $0.10/0.20=0.5$. These express different scales of comparison, and neither alone expresses its uncertainty. Assignment, follow-up and the definition of the event determine what effect can be estimated. For incomplete follow-up, methods from <survival analysis> distinguish observing no event yet from establishing that an event will never occur.
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