Robbery can vary from stealing from organizations to personal property, where the latter is often termed 'mugging', 'snatch theft' or 'street robbery'. Many studies that explore robbery do so in terms of variations in the frequencies of offence characteristics. Further, some explore the combinations of these characteristics and propose models to explain the psychological meaning of such patterns of variation. Alison reported three themes of robberies, termed 'cowboys', 'bandits' and 'Robin's men' based upon the defining features of each set of behaviours. The main motivation for robbery is a need for fast cash but the 'decision' to rob is influenced by 'street culture'. For armed robbery (commercial), it has been suggested that co-offending has an instrumental advantage, with the number of offenders allowing role differentiation. However, regardless of motive or target, robbery tends to be a group phenomenon, with group patterns evident even in the least skilled or planned crimes.