Mid year population: Because the population size changes daily due to births, deaths and migration, the mid year population is commonly chosen as a denominator. The population as on 1st July is mid-year population.
Population at risk: It is important to note that the calculation of measures of disease frequency depends on correct estimates of the numbers of people under consideration. Ideally, these figures should include only those people who are potentially susceptible to the disease studied. For instance, men should not be included in denominator for the carcinoma of cervix.Part of population, which is susceptible to a disease is called the population at risk, e.g., Occupational injuries occur only among working people so the population at risk is the workforce.
Person – time: In some epidemiological studies (e.g. cohort studies), person may enter into the study at different times. Consequently, they are under observation for varying time period. In such case, the denominator is a combination of person and time. The most frequently used person time is person- years. Some times this may be person months, person weeks or man- hours. For example, if 10 persons were observed in the study for 10 years, person time would be 100 person years of observation. The same figure would be derived if 100 persons were under observation for one year. These denominators have the advantage of summarizing the experience of persons with different duration of observation or exposure.
