situation, the paradoxical situation may occur that this medication would be positively associated with the prevalence of dementia, and so be misconstrued as a causative agent. For this reason incidence, rather than prevalence, is the desired measure of disease frequency. INCIDENCE Incidence refers to the number of new cases over a given period of time. The observed number of new cases depends on the evaluated duration of follow up. To be able to compare studies with varying duration of follow up, incidences per year are usually given. Furthermore, within a given study, length of follow up time may differ between subjects. To profit from all the available information, the length of time at risk is determined for every person. The total length of follow up time is obtained after summing all person-times, and represented as the number of person-years of follow up. Most studies on incidence report incidence rates that are calculated as the number of new cases divided by the person-years at risk. Incidence rates are usually represented as number of new cases per 1000 person-years. Pooled estimates of incidence In the same collaborative effort that pooled prevalence data of European studies, data on incidence of dementia of eight population based European studies were compared and pooled.6 In total, there were 42 996 person-years of follow up with 835 new dementia cases. Of these, 60–70% were diagnosed with AD and 15–20% with VaD. Incidence rates of dementia increased exponentially with age from 2.4 per 1000 person-years in the 65–69 age group, to 70.2 per 1000 personyears in the 90+ age group. Rates among women were higher, especially above the age of 80 (fig 5). The rates continued to increase with age in women, whereas the increase reached a plateau in men at age 85. For AD, findings were comparable, with pooled incidence rates increasing from 1.2 per 1000 person-years among 65–69 year olds to 53.5 among subjects over 90 years old. Will we be all demented at the age of 140? The question of whether the incidence rates reach a plateau at a certain age is important, as an exponential increase in Figure 4 Working age population and number of working age persons per one demented person( = ratio) in Europe using age and sex adjusted mean numbers of prevalent cases. Based on Wancata et al. 5 Figure 5 Pooled incidence rates of dementia by sex. Based on Fratiglioni et al. 6 v4 NEUROLOGY IN PRACTICE www.jnnp.com copyright. on December 4, 2021 by guest. Protected by http://jnnp.bmj.com/ J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.2005.082867 on 16 November 2005. Downloaded from incident AD would suggest that the disease is an inevitable consequence of aging, whereas convergence to a fixed value or a decline could suggest that an element of the population has reduced vulnerability, owing perhaps to genetic or environmental factors.7 Results with respect to incidence increasing with age have been conflicting, with some studies suggesting an ongoing increase with advancing age, whereas other studies suggest that incidence rates reach a plateau after a certain age. This issue is difficult to resolve, however, as the oldest age groups are always underrepresented, resulting in less precise estimations. The Cache County in Utah, USA, is known for the longevity of its inhabitants. The relatively large proportion of extremely old individuals provides the opportunity to give reliable estimates of incident dementia among the oldest old. There were 185 new cases of dementia (123 AD) among 3308 participants who contributed 10 541 person-years of follow up.7 The incidence of dementia increased with advancing age from 2 per 1000 person-years in the group aged ( 68, to peak with 122 per 1000 person-years in the 90–92 age group, and decline in the 93+ age group (110 per 1000 person-years). The incidence of dementia was higher in females over the age of 80. If incidence rates would indeed plateau at a certain age, then the future public health burden of dementia and AD, albeit still enormous, might be less than previously projected. Variation across regions? Incidence rates have been found to vary between studies. Methodological issues partly account