Epidemiology

Epidemiology

A: What are the definitions for incidence and prevalence rate? Identify the differences.

Rates are presented as number of observations per 1000 person (patient) years of observation. A patient-year starts at the beginning of an observation frame or when a patient registers in the practice, and is closed when the patient leaves the practice for any reason, including death. Where patients are not registered with the FD, but tend to see the same FD for most, but possibly not all, healthcare problems, a patient-year was opened when a patient presented to the FD for an encounter. Any patients who did not consult in the observation period of five years did not contribute to the denominator. Incidence rates give the rate of an observation i.e. at the first encounter at the start of a new per 1000 patient years of observation. Prevalence rates give the rate of an observation in all both incident and rest-prevalent considered together, in that period of observation. The incidence to prevalence rate ratio could be used as an index to define a chronic condition as one with a low ratio, below a defined 'cut-off' threshold level and independent of a specific duration period. Thresholds of 20, 30 or 50% would identify different sets of conditions as chronic, and others as not (James, 1999).

Rest-prevalent rates represent a health problem that is not new, but has presented during that period of observation for follow-up. For instance, the association between ethnicity and illness is then treated like that between carrying a cigarette lighter and developing lung cancer. People who carry a lighter are often smokers, but controlling for smoking will quickly show that lighters themselves do not cause cancer. In this example, smoking is the confounder which produces the misleading correlation between carrying a lighter and developing lung cancer. However, ...