-Epidemic duration. The vast majority of seasonal influenza outbreaks are observed between the extremes of mid-November and the end of April [11] . Since a background rate of ILI incidence is present throughout the year, duration is difficult to define precisely. The range of values quoted for the duration of an annual epidemic (described with the acronym ADE in this paper) is typically between three and 16 weeks [1, 16] . -Attack rate. The proportion of the population infected in a year, which we term the AAR, is very difficult to determine, as a significant proportion of infections are asymptomatic and only a proportion of symptomatic cases seek healthcare. Hence sentinel data on ILI can only give relative information, such as the fractional variation in incidence over time. In addition, measures such as ILI are non-specific for influenza and can be caused by a variety of respiratory pathogens. That said, ILI data from France and the UK indicate a standard deviation for AAR across successive years of approximately 40 per cent of the mean attack rate [17, 18] . Results from serological and virus isolation data from closely monitored populations indicate an AAR range of approximately 10-20% for seasonal influenza, rising to 30 per cent or more for influenza pandemics [1, 17] . Information on consultation rates for known cases also suggests a mean AAR of around 15 per cent [13, 18, 19 ]. -Periodic behaviour. Seasonal influenza is characterized by annual outbreaks in temperate countries in the sense that there is almost invariably a marked seasonal increase in case rate during the winter months. However, it cannot be said to be periodic in the sense that successive outbreaks are comparable in magnitude and form with each other. In this sense, seasonal influenza has no clear periodicity (annual, biennial and triennial) and contrasts with the behaviour of diseases such as measles, which has a pronounced biennial structure in the pre-immunization period [5] . The metrics that we use to compare model and time-series behaviour are able to pick up this characteristic aperiodicity. -Strain variation. Virus isolation studies show that individual seasonal epidemics are usually dominated by a single type and/or subtype, although others may be present at low levels [13] . Successive seasons are usually dominated by different types and subtypes, although often the same strain is present for several seasons (figure 1).