1146 J Neurol (2011) 258:1140–1149 Fig. 3 MS in UKI immigrants to Four States of Australia by age at immigration and years from immigration to clinical onset in Group II (entry 1947?) migrants their ages at immigration, their length of stay in their new land, and their ages at prevalence day [14]. The true pop- ulation at risk according to age at survey and age at migration and age at clinical onset can be very difﬁcult to deﬁne, and the choice of denominator or the type of rate required can get quite involved. The desired population denominator is generally not available from routine sources [15]. Here we were able to estimate the speciﬁc risk of developing MS by age at immigration for the UKI immi- grants, by restricting both numerator and denominator to 123