In science, a half-life (also, as a noun, spelled half life ) is the amount of time it takes for half of a substance or entity to undergo some specified process. For example, the half-life of a radioactive substance is the amount of time it takes for half of its atoms to decay, and the half-life of a drug is the amount of time it takes before half of the active elements are either eliminated or broken down by the body. Depending on the particular substance involved, a half-life may be significantly shorter or longer than half of the substance's full period of activity. The concept of a half-life has been adopted by fields outside of science, such as marketing and finance.
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.
In finance, the half-life of a mortgage is the date when half of the principal amount has been paid. Because of the way that interest is pro-rated (as a proportion of the total, paid more towards the first part of the life of the mortgage), the mortgage half-life date is generally more distant than the chronological half-way point in the total life of the mortgage.