Gibrat's law

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Gibrat's law, sometimes called Gibrat's rule of proportionate growth is a rule defined by Robert Gibrat (1904-1980) stating that the size of a firm and its growth rate are independent. Gibrat's law is also applied to cities size and growth rate as well, where proportionate growth process may give rise to a distribution of city sizes satisfying Zipf's law.

In general, processes characterized by Gibrat's law converge to a limiting distribution, which may be log-normal or power law, depending on more specific assumptions about the stochastic growth process.

In the study of the firms (business), the scholars do not agree that the foundation and the outcome of Gibrat's law are empirically correct.