doubling time: one construct, many systems

In microbiology the doubling time (or generation time) is the interval in which a growing population of cells doubles in number during balanced exponential growth, set by the specific growth rate of the culture. Monod's 1949 The growth of bacterial cultures formalised how that growth rate, and hence the doubling time, depends on nutrient concentration. The referent is the characteristic timescale of exponential increase in a cell population, the same quantity that measures how fast a human population or an epidemic grows.[1]

Unit

The time for a cell population to double in number under exponential growth

Recovery means

Not applicable

Measured by

Computed as ln 2 divided by the specific growth rate, estimated from the slope of log abundance against time

In demography the doubling time is how long a population takes to double at its current growth rate, a compact way to express the speed of population increase (the familiar "rule of 70" approximates it by dividing 70 by the percentage growth rate). Preston, Heuveline, and Guillot's Demography: Measuring and Modeling Population Processes derives it from the exponential growth model. It is the identical exponential-doubling construct as in a bacterial culture, applied to people rather than cells.[2]

Unit

The number of years for a population to double at its current growth rate

Recovery means

Not applicable

Measured by

Approximated as ln 2 divided by the growth rate r, or by the rule of 70 applied to the percentage growth rate

In epidemiology the doubling time is the interval in which the cumulative number of cases doubles during the early, roughly exponential phase of an outbreak, a key early signal of how fast an epidemic is spreading and a quantity closely tied to the reproduction number. Zhou and colleagues' 2020 COVID-19 seeding time and doubling time model used it as an early risk-assessment tool during the pandemic. It is the same exponential-doubling measure as in microbial or population growth, read off case counts instead of cells or people.[3]

Unit

The time for cumulative cases to double in the early exponential phase of an outbreak

Recovery means

Not applicable

Measured by

Estimated from the exponential growth rate of early case counts (ln 2 divided by the fitted growth rate)