K-factor (marketing)

In viral marketing, the K-factor can be used to describe the growth rate of websites or apps. The formula is roughly as follows:

$i=\text{number of invites sent by each customer }$ (e.g. if each new customer invites five friends, i = 5)

$c=\text{percent conversion of each invite }$ (e.g. if one in five invitees convert to new users, c = .2)

$k = i * c$

This usage is borrowed from the medical field of epidemiology in which a virus having a k-factor of 1 is in a "steady" state of neither growth nor decline, while a k-factor greater than 1 indicates exponential growth and a k-factor less than 1 indicates exponential decline. The k-factor in this context is itself a product of the rates of distribution and infection for an app (or virus). "Distribution" measures how many people, on average, a host will make contact with while still infectious and "infection" measures how likely a person is, on average, to also become infected after contact with a viral host.