J Neurol (2011) 258:1140–1149 Methods Previous publications [9–12] have described the methods of case ascertainment and diagnosis used to estimate the prevalence of MS in Australia on prevalence day, 30 June 1981, in the states of New South Wales (NSW), Queens- land (QLD), South Australia (SA) and Western Australia (WA), called in this paper the four states. In each survey approximately 70% of cases were classed as clinically deﬁnite MS, with 20% as probable MS and 10% possible MS, according to the diagnostic criteria of Rose et al. [13]. In the present analysis Hobart in Tasmania, included in the previous study [1], was omitted as its high prevalence rate would preclude any differential risk for UKI migrants. In that report the analysis had been restricted to cases aged 20–49 years in 1981 because of limitations in the available census data. For this presentation, the original data sheets from the study were reassessed. Each MS case had been described by case number; sex; state of residence; country of origin within UKI; ages at migration, 1981 and onset; and dura- tion from immigration to 1981. From these, calendar years of birth, immigration and onset were calculated. All data for each of the four states and their total were evaluated. As with prior assessments, all immigrants with symptoms of MS before immigration were excluded from consideration. Population denominators for the UKI immigrants to each of the four states had been provided as special cross- tabulations from the 1981 Census of Population and Housing by the Australian Bureau of Statistics. These listed on the y axis numbers of immigrants by duration of resi- dence in Australia for single years 0–34 and for 35? years, and on the x axis age groups for residents age 0–79 and for age 80? in 1981. From these tables, population numbers for each individual age at immigration were calculated for this reassessment. This was carried out by ﬁrst listing single years of age in 1981 as the average for that particular age group for ages 0–79. The ﬁrst row, giving age in 1981 for those with \1 year of residence, was thus also the age at immigration for that speciﬁc set of immigrants. By then shifting each consecutive row comprising the 1981 age distributions for those with 1, 2, 3… years duration of residence by 1 year to the left, each displaced row then provided the age at immigration for migrants with that speciﬁc duration in Australia. For example, persons age 17 in 1981 who had a duration of residence in Australia of 1 year would have been age 16 at immigration. The earliest calendar year of immigration was therefore 1947 for those who had entered 34 years before prevalence day. Summing each new column then provided the population distribution for age at immigration by single years of age from age 0 (= age \1) through age 79 for those UKI immigrants who had entered Australia in 1947 or later and had survived as 1141 residents of the cited state into 1981. After collecting them into 5 year age groups, this provided the denominator to calculate risk ratios per 100,000 population for MS at each age at immigration. Note that this is not a prevalence rate or ratio, but rather an absolute risk ratio for this speciﬁc population at risk. To calculate here the absolute risk of MS by age at immigra- tion, the speciﬁc population at risk of MS required is (1) the number of UKI immigrants by age at immigration for each of the four states, (2) who had entered Australia from 1947 to 1981, and (3) who had survived at age 0–79 years on prevalence day in 1981. The numerator by age at immigration for this denominator therefore had to be lim- ited to that same speciﬁc subset of the MS cases who had the same characteristics of year of entry (1947?), and survival; this is called Group II. The remainder (Group I) were separately assessed and will be the subject of a later report. In order to place both groups in context and explore further reasons for the occurrence of MS within them, population numbers for each state and census, 1901–1981, were ascertained from Australian Historical and Population Statistics on AusStats (3105.0.65.001). The total resident population for each state at each census is divided into those born in Australia and those born overseas, the latter by speciﬁc country of birth. The immigrant populations by state of residence in Australia and country of origin (birthplace) were collected and combined into UKI migrants and migrants from other high MS risk countries, as cited in Table 1 below. Immigrants from the remaining countries were not considered. Standard descriptive statistics were carried out. Poisson 95% conﬁdence intervals were calculated for each rate or ratio. Age adjustment of rates to the US 1960 population by the direct method was performed, as perhaps giving more distinct comparisons than adjustment to an Australian base. Results The denominator In Fig. 1 is a map of the Commonwealth of Australia by state and territory, with the major cities identiﬁed. Can- berra, the national capital, lies in the Federal Capital Ter- ritory, and is not part of the surrounding state of New South Wales. Populations of Australia and the four states of the study were available for each census year, 1901, 1911, 1921, 1933, 1947, 1954, 1961, 1971, and 1981. In this interval the Commonwealth had grown from under 4 mil- lion residents to almost 15 million. The percentage of native-born Australians had risen from 77% in 1901 to 90% in 1947, suggesting little immigration in that interval. 123