1. A mathematical formula (model) that relates one biological factor to another. Once a mathematical relationship is established, scientists use the model to predict one factor over another; 2. A statistical procedure similar to an analysis of variance or a multiple regression that is used to estimate the magnitude of the effects of different factors on a variable of interest. GLM's are the tool of choice for standardising catch per unit effort (CPUE) data in order to obtain indices of abundance. In such applications, the variable of interest is CPUE and the factors are year and perhaps others such as area, gear configuration, etc. ; the standardised abundance index of annual abundance would then be given by the parameters associated with the factor, year.