<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (February 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 B with respect to A is 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 . </P> <P> Fold change is so - called as 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's Dictionary of Mathematics, define "- fold" to mean "times," 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 .". More 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". </P> <P> Fold change is often used in analysis of gene expression data from microarray and RNA - Seq experiments for measuring change in the expression level of a gene . A disadvantage and serious risk of using fold change in this setting is that it is biased and may misclassify differentially expressed genes with large differences (B-A) but small ratios (B / A), leading to poor identification of changes at high expression levels . Furthermore, when the denominator is close to zero, the ratio is not stable and the fold change value can be disproportionately affected by measurement noise . </P>

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