Calculating the level population of an ion in collisional ionization equilibrium is conceptually quite simple. After calculating the level population for each ion, simply multiply the level population by the radiative transition rate (also known as the Einstein 'A' value) for each transition of interest. Calculating the level population is somewhat more difficult but still straightforward. However, it requires the use of a number of equations and so is described in the following memo. This is the approach used by APEC for all level population calculations. Many thanks to Yuanzhen Han (Purple Mountain Observatory, Chinese Academy of Sciences) for correcting some errors in the original version of the memo.