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Fisk Distribution (Log-Logistic): Definition

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The Fisk distribution, also called the log-logistic distribution, is a continuous probability distribution with many applications: from modeling the distribution of wealth or income in economics, to stream flow rates in hydrology to the lifetime of an organism in biostatistics. It is especially useful in modeling situations where the rate something is happening increases initially, and then after some time begins to decrease.

The term ‘fisk distribution’ is primarily used in economics; in other fields, ‘log-logistic distribution’ is more common.

The fisk distribution has two parameters, a scale parameter and a shape parameter. Both are positive numbers. To say that a random variable x has the log-logistic distribution, we write x ~ loglogistic(λ κ). Here λ is the scale parameter and κ is the shape parameter; both, again, are positive.

The Fisk Distribution’s Probability Density Function

The probability distribution function (pdf) is given by:

fisk distribution pdf

where λ is the scale parameter and κ is the shape parameter.

Other Important Functions

The cumulative distribution function, on the support of X, is given by:
fisk cdf

The survival function, on the support of X, is given by:
fisk survivor function

The population mean and variance, for a data set that follows the fisk distribution, is given by:
fisk dist mean variance

The median of x is just 1/λ.


Al-Shomrani, Shawky, Arif & Aslam. Log-logistic distribution for survival data analysis using MCMC. SpringerPlus (2016) 5:1774. DOI 10.1186/s40064-016-3476-7. Retrieved from on May 18, 2018

Leemis, Larry. Log Logistic Distribution. Retrieved form on May 18, 2018

World Heritage Encyclopedia, Fisk Distribution. Retrieved from on May 6, 2018

Stephanie Glen. "Fisk Distribution (Log-Logistic): Definition" From Elementary Statistics for the rest of us!

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