Frequency, incidence, and prevalence

I have read with interest the paper “Recent incidence and descriptive epidemiological survey of breast cancer in Saudi Arabia” published by Saggu et al1 in the October 2015 issue of Saudi Medical Journal. In this paper, the authors reported a frequency profile of breast cancer (BC) in the Kingdom of Saudi Arabia (KSA) for almost 2 decades indicating that the cancer increased over time in this region of the Middle East. It does not, however, seem to be realistic to conclude a decreasing or increasing time trend based on these absolute figures derived from the Saudi Cancer Registry (SCR) and King Faisal Specialist Hospital and Research Centre in Riyadh, KSA. In epidemiological studies, time trends are measured using prevalence and incidence rates rather than frequencies. Incidence measures the risk probability of health conditions (such as, diseases, and so forth) in a population within a specified period of time. This is the number of new cases within a specified time period divided by the size of the population initially at risk of the condition (such as, diseases, and so forth). For instance, if a population initially contains 100,000 nondiseased (but at risk) individuals with 120 persons who develop the condition over a year while they were under the risk, the incidence is 1.2 cases (per 1000 persons) in this population. The concept and measurement of prevalence rate is, however, quite different of incidence. The prevalence is the proportion of all cases, either new or previous ones, in a population at a given time. Thus, the incidence rate is the risk of contracting the disease, whereas the prevalence shows the magnitude of the disease.2