Measures of Disease Frequency

The occurrence of disease has three measurements. These are prevalencePrevalence, risk, and incidence. They differ by how they relate to time. Prevalence addresses a point in time, risk addresses a period of time and incidence is a function of time. Both prevalence and risk are proportions, so they have similar methods of taking chance into account. Both have the binomial distributionBinomial distribution as their sampling distributionSampling distribution. Both have approximate and exact interval estimates. Incidence, on the other hand, is a rateRate instead of a proportionProportion. A characteristic of rates is that they have units of measurement of 1/time. When we estimate incidence, we must determine how long each person was at risk. In doing this, we use the actuarial method which assumes a person who got the disease did so halfway through their last period of follow-up. When we take chance into account for rates, we focus on the numerator (number of events) and ignore the denominator (time). This gives us the Poisson distributionPoisson distribution sampling distributionSampling distribution. Incidence has both approximate and exact interval estimates.