Relationship between prevalence and incidence

P / (1-P) = IR x D
Prevalence depends on incidence rate and duration of disease (duration lasts from onset of disease to its termination)
• If incidence is low but duration is long - prevalence is relatively high
• If incidence is high but duration is short - prevalence is relatively low
• Conditions for equation to be true: Steady state, IR constant, Distribution of durations constant
If the prevalence of disease is low (less than 10%), the equation simplifies to P = IR x D
Figuring duration from prevalence and incidence:
Lung cancer incidence rate = 45.9/100,000 py
Prevalence of lung cancer = 23/100,000
D = P /IR = 23/100,000 p / 45.9/100,000 py =0.5 years
Conclusion: Individuals with lung cancer survive 6 months from diagnosis to death