The Reed–Muench method is a simple method for determining 50% endpoints in experimental biology,[1] that is, the concentration of a test substance that produces an effect of interest in half of the test units. Examples include LD50 (the median lethal dose of a toxin or pathogen), EC50 and IC50 (half maximal effective or inhibitory concentration, respectively, of a drug), and TCID50 (50% tissue culture infectious dose of a virus).
The reason for using 50% endpoints is that many dose-response relationships in biology follow a logistic function that flattens out as it approaches the minimal and maximal responses, so it is easier to measure the concentration of the test substance that produces a 50% response.
https://web.archive.org/web/20131119121647/http://aje.oxfordjournals.org/content/27/3/493.extract