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Ideally, morbidity rates should be calculated using a population denominator, just like mortality rates are. This then corrects for any changes in the population. As a result, an increase in the size of the population which produces an increase in the overall number of cases of disease does not appear to be a change in the risk of disease.
Burundian refugee camps in western Tanzania, 1997
(Source: International Rescue Committee)
This graph shows the number of cases of malaria reported from health facilities per 1000 population per week during the first 7 months of 1997. It is readily apparent that the incidence of malaria increased during this time period and that the rate is about the same in children less than 5 years of age and older children and adults. Combining these data with those above from the same population, one can conclude that the case-fatality ratio for malaria is much higher in young children than older children and adults.
However, it is sometimes impossible to get even a rough estimate of the population size. In such cases, plotting the absolute number of cases can give you some idea of the trend if the size of population from which the cases are counted is not changing rapidly.
Goma, Zaire 1994
This graph shows only the number of cases of diarrhoea reported from health facilities in the three camps around Goma, Zaire. The population denominator was unknown. However, there was little emigration or immigration in these camps at this time. The major cause of change in the population size was the substantial mortality during this cholera and dysentery outbreak.