COMPUTER-SIMULATION ANALYSIS OF SARTWELLS INCUBATION PERIOD MODEL FORDISEASES WITH UNCERTAIN ETIOLOGY - THE EFFECT OF COMPETING RISK

Computer simulation was applied to Sartwell's model to examine the imp act of competing risks of death on the underlying assumptions and the power to reject both uniform and normal incubation period distribution s. Exponential and nonparametric survival functions were imposed onto lognormal, uniform, and normal distributions to create random samples reflecting competing risk. These random samples were evaluated with th e Shapiro-Wilk's test to determine the proportion for which the lognor mal distribution was rejected. The simulations indicated that competin g causes of death do not significantly alter the lognormal distributio n of incubation periods. In only approximately 5% of the samples drawn from a lognormal distribution was a lognormal hypothesis rejected wit h a goodness-of-fit test when sample size varied from 20 to 500. There was generally good power (>80%) to reject a lognormal distribution if the random samples were generated from a uniform distribution of incu bation times, but not when they were generated from a normal distribut ion, particularly with increasing ages at disease onset. Varying the s tandard deviation did not significantly change the simulation results if the random samples came from a lognormal or uniform distribution. T hese conclusions were further supported by application of Sartwell's m odel to published data on the ages of onset for several chronic diseas es.