The ‘quick and dirty’ method

If a disease is rare, it is possible to make a rough estimate of the cumulative incidence by adding up the incidence rates for each year of life from 0 to 74. Since incidence rates are usually presented for 5-year age groups, e.g. 0–4 years, 5–9 years, etc., the rate at age 0 is the same as that at ages 1, 2, 3 and 4 years; similarly the rate at age 5 is the same as that at ages 6, 7, 8 and 9 years; and so on for each 5-year age-group. This means that, if the incidence in a 5-year band is 3/100,000, the chance a person develops disease during one of the 5 years is 3/100,000 and it is 15/100,000 for the whole 5-year period. One way to add up all the incidence rates to age 74 is therefore to multiply each of the age-specific rates by 5 (assuming that they are for 5-year age groups) and then to add them up. Or, to save time, you can do it the other way around and add up the 5-year rates and then multiply by 5 to obtain the same answer.