Fold Change

출처: http://en.wikipedia.org/wiki/Fold_change

Fold change is a measure describing how much a quantity changes going from an initial to a final value. For example, an initial value of 30 and a final value of 60 corresponds to a fold change of 2, or in common terms, a two-fold increase. Fold change is calculated simply as the ratio of the final value to the initial value^{[citation needed]}, i.e. if the initial value is A and final value is B, the fold change is B/A. As another example, a change from 80 to 20 would be a fold change of 0.25, while a change from 20 to 80 would be a fold change of 4. Some practitioners replace a fold-change value that is less than 1 by the negative of its inverse^{[citation needed]}, e.g. a change from 80 to 20 would be a fold change of -4 (or in common terms, a four-fold decrease).

A benefit of expressing a change as the ratio between an initial value and a final value -- a fold change -- is that the change itself is emphasized rather than the absolute values. This property makes the fold change suitable for statistical tests that need to normalize data to eliminate systematic error. The distributional fold change test is based upon this idea.

Fold change is often used in analysis of gene expression data in microarray and RNA-Seq experiments, for measuring change in the expression level of a gene. ^[1]