Five-year age-specific incidence rates I Their nature and limitations

SUMMARY The effect of population structure on five-year age-specific incidence rates was investigated using the one-year population data from life tables and a theoretical age incidence curve of the form: I = btk where I is the incidence at age t, and b and k are constants. The five-year incidence rates differed systematically from the one-year rates of the central year of the five-year period. This difference depended on the change with age of both the population size and the incidence rate. Thus at ages 20-24 the five-year rate overestimates the mid-period one-year rate by about 4%, but the overestimate progressively decreases to become an underestimate of 0O5 % at ages 75-79. In consequence the one-year and five-year rates produce fitted age incidence curves with different slopes; the value of k in the incidence equation is about 07 % greater for the one-year rates. The population structures of developed and underdeveloped countries are markedly different and these were found to affect the five-year incidence rates, but never by more than 0'5%. The effect of the irregularities in one-year age structure of real populations on the observe