Fold change is a measure describing how much a quantity changes between an original and a subsequent measurement. It is defined as the ratio between the two quantities; for quantities A and B, then the fold change of with respect to B/A. Fold change is often used when analysing multiple measurements of a biological system taken at different times as the change described by the ratio between the time points is easier to interpret than the difference. Fold change is so called because it is common to describe an increase of multiple X as an X-fold increase. As such, several dictionaries, including the Oxford English Dictionary and Merriam-Webster Dictionary, as well as Collins. Dictionary of Mathematics, define fold to mean time, as in 2-fold 2 times double. Likely because of this definition, many scientists use not only fold but also fold change to be synonymous with times, as in 3-fold larger 3 times larger are ambiguous is fold decrease, where, for instance, a decrease of 50% between two measurements would generally be referred to a half-fold change rather than a 2-fold decrease.
