Logistic Distribution

Logistic Distribution is a common form of distribution and has many uses. Its main use is for representing data over time, or categorical dependent variables. Categorical dependent variables are most commonly linked to yes/no choices or a question with multiple answers. It is also used to show the growth of populations in comparison to one another, or the rise and fall of income. In a more interesting development, the United States Chess Federation has only recently changed the use of a normal distribution for calculating scores to a logistic distribution.
This function is mostly easily described through F(x) = exp(x) / (1 + exp(x)).

Sources:
http://www.johndcook.com/blog/2010/05/18/normal-approximation-to-logistic/
http://en.wikipedia.org/wiki/Logistic_distribution