In addition, with any phylogenetic study such as this, there is always a limitation on the number of sequences that are available (i.e. the number of respiratory samples containing influenza viruses that have been collected and sequenced in any one influenza season), and the number that can be comfortably analyzed within a given time-frame (i.e. the limitations on computing power, which again may be more of a problem in tropical and subtropical countries that are more resource-limited).