1. A computer-implemented method of generating a model of a group of persons or animals, such that each of the persons or animals has at least one attribute, comprising: generating individual models of units of the group, each unit representing a person or animal; storing a set of possible attributes to be associated with the units; displaying a list of the attributes, the display further having a minimum value entry mechanism and a maximum value entry mechanism associated with each attribute; receiving data from a user, via the minimum value entry mechanism and the maximum value entry mechanism, representing a group-level range of values for a selected attribute; displaying at least one distribution entry mechanism, operable to receive input from the user representing a desired distribution type of attribute intensity values; receiving data from the user representing the desired distribution type; calculating an intensity value of the attribute for units of the group, based on the desired distribution type, such that the units have intensity values with a distribution among the group consistent with the desired distribution type and within the group-level range; and displaying a plurality of the units as icons having a graphical feature representing the selected attribute.