In physics , a fixed time required for half the radioactive nuclei in a substance to decay. Half-lives of radioactive substances can range from fractions of a second to billions of years, and they are always the same for a given nucleus, regardless of temperature or other conditions. If an object contains a pound of a radioactive substance with a half-life of fifty years, at the end of that time there will be half a pound of the radioactive substance left undecayed in the object. After another fifty years, a quarter-pound will be left undecayed, and so on.
In marketing, half-life is a formula for estimating the total responses to be expected from a direct marketing campaign. The half-life of a catalog is the day when half of the consumer responses to the mailing have been received. Sears Canada studied the history of their sales data and found that their catalogs reached their half-life 20 days after being mailed, although the life of the catalog was actually several months. Knowing when a direct marketing campaign reaches the half-life point allows marketers to adapt more quickly to consumer demand.